Data transmission method in wireless communication system and device therefor

ABSTRACT

Disclosed is a downlink (DL) multi-user (MU) transmission method of an AP device in a wireless local area network (WLAN) system including generating a DL MU physical protocol data unit (PPDU) including a physical preamble and a data field, the data field including at least one MAC protocol data unit (MPDU), the at least one MPDU including a MAC header and a MAC frame body, and the MAC header including ACK indication information for uplink (UL) MU transmission of an ACK frame as a response to data transmitted through the data field, and transmitting the DL MU PPDU.

TECHNICAL FIELD

The present invention relates to wireless communication systems, andmore particularly, to a method for transmitting data for supporting adata transmission of multi-user and a device for supporting the same.

BACKGROUND ART

Wi-Fi is a wireless local area network (WLAN) technology which enables adevice to access the Internet in a frequency band of 2.4 GHz, 5 GHz or60 GHz.

A WLAN is based on the institute of electrical and electronic engineers(IEEE) 802.11 standard. The wireless next generation standing committee(WNG SC) of IEEE 802.11 is an ad-hoc committee which is worried aboutthe next-generation wireless local area network (WLAN) in the medium tolonger term. IEEE 802.11n has an object of increasing the speed andreliability of a network and extending the coverage of a wirelessnetwork. More specifically, IEEE 802.11n supports a high throughput (HT)providing a maximum data rate of 600 Mbps. Furthermore, in order tominimize a transfer error and to optimize a data rate, IEEE 802.11n isbased on a multiple inputs and multiple outputs (MIMO) technology inwhich multiple antennas are used at both ends of a transmission unit anda reception unit.

As the spread of a WLAN is activated and applications using the WLAN arediversified, in the next-generation WLAN system supporting a very highthroughput (VHT), IEEE 802.11ac has been newly enacted as the nextversion of an IEEE 802.11n WLAN system. IEEE 802.11ac supports a datarate of 1 Gbps or more through 80 MHz bandwidth transmission and/orhigher bandwidth transmission (e.g., 160 MHz), and chiefly operates in a5 GHz band.

Recently, a need for a new WLAN system for supporting a higherthroughput than a data rate supported by IEEE 802.11ac comes to thefore.

The scope of IEEE 802.11ax chiefly discussed in the next-generation WLANtask group called a so-called IEEE 802.11ax or high efficiency (HEW)WLAN includes 1) the improvement of an 802.11 physical (PHY) layer andmedium access control (MAC) layer in bands of 2.4 GHz, 5 GHz, etc., 2)the improvement of spectrum efficiency and area throughput, 3) theimprovement of performance in actual indoor and outdoor environments,such as an environment in which an interference source is present, adense heterogeneous network environment, and an environment in which ahigh user load is present and so on.

A scenario chiefly taken into consideration in IEEE 802.11ax is a denseenvironment in which many access points (APs) and many stations (STAs)are present. In IEEE 802.11ax, the improvement of spectrum efficiencyand area throughput is discussed in such a situation. More specifically,there is an interest in the improvement of substantial performance inoutdoor environments not greatly taken into consideration in existingWLANs in addition to indoor environments.

In IEEE 802.11ax, there is a great interest in scenarios, such aswireless offices, smart homes, stadiums, hotspots, andbuildings/apartments. The improvement of system performance in a denseenvironment in which many APs and many STAs are present is discussedbased on the corresponding scenarios.

In the future, it is expected in IEEE 802.11ax that the improvement ofsystem performance in an overlapping basic service set (OBSS)environment, the improvement of an outdoor environment, cellularoffloading, and so on rather than single link performance improvement ina single basic service set (BSS) will be actively discussed. Thedirectivity of such IEEE 802.11ax means that the next-generation WLANwill have a technical scope gradually similar to that of mobilecommunication. Recently, when considering a situation in which mobilecommunication and a WLAN technology are discussed together in smallcells and direct-to-direct (D2D) communication coverage, it is expectedthat the technological and business convergence of the next-generationWLAN based on IEEE 802.11ax and mobile communication will be furtheractivated.

DISCLOSURE Technical Problem

An object of the present invention is to propose an uplink/downlinkmulti-user data transmission and reception method in a wirelesscommunication system.

Another object of the present invention is to propose a high efficiency(HE) format of a PPDU used in uplink/downlink multi-usertransmission/reception in a wireless communication system. Inparticular, the format of the HE-SIG (signal) A field and HE-SIG B fieldincluded in the PPDU is proposed.

The technical objects of the present invention are not limited to thoseobjects described above; other technical objects not mentioned above maybe clearly understood from what are described below by those skilled inthe art to which the present invention belongs

Technical Solution

According to an aspect of the present invention, there is provided anaccess point (AP) device of a WLAN system and a data transmission methodof an AP device.

A downlink (DL) multi-user (MU) transmission method of an AP device in awireless local area network (WLAN) system may include: generating a DLMU physical protocol data unit (PPDU) including a physical preamble anda data field, the data field including at least one MAC protocol dataunit (MPDU), the at least one MPDU including a MAC header and a MACframe body, and the MAC header including ACK indication information foruplink (UL) MU transmission of an ACK frame as a response to datatransmitted through the data field; and transmitting the DL MU PPDU.

The at least one MPDU may include an indicator indicating that the atleast one MPDU includes the ACK indication information.

The indicator may be included in an MPDU delimiter field included in theat least one MPDU.

When a type or a sub-type of the at least one MPDU is defined, theindicator may be included in a frame control field of the MAC header, asthe defined type or sub-type.

When the at least one MPDU is a control wrapper frame in ahigh-throughput (HT) format, the indicator may be included in an HTcontrol field included in the MAC header.

When the at least one MPDU is a high-efficiency (HE) format frame, theindicator may be included in an HE control field included in the MACheader.

When a specific reserved bit value of an MPDU delimiter fieldcorresponding to the at least one MPDU is set to a preset value or whena specific reserved bit value of a control field included in the atleast one MPDU is set to a preset value, the at least one MPDU may bethe HE format frame.

The indicator may be included in a frame control field or an addressfield included in the MAC header.

When the indicator is included in the frame control field, bit values ofa To DS field and From DS field of the frame control field may each beset to 1.

The ACK indication information may be included in a control field of theMAC header.

The control field may include at least one of frequency resourceallocation information, bandwidth information, space resource allocationinformation, transmission channel information, modulation and codingscheme (MCS) level information, maximum length information of a UL MUPPDU carrying the ACK frame, buffer status report request information,and channel status report request information, as the AK indicationinformation, for UL MU transmission of the ACK frame.

When the data field includes an A-MPDU, a portion of the A-MPDU mayinclude the at least one MPDU.

The DL MU transmission method may further include: when the ACK frame isnot received as a response in accordance with the ACK indicationinformation, transmitting a block acknowledgement response (BAR) frameto a reception STA of the ACK indication information, receiving arequest for transmission of the BAR frame from the reception STA andtransmitting the BAR frame to the reception STA in response to therequest, or re-transmitting data corresponding to the reception STA tothe reception STA through the DL MU PPDU.

The transmitting of a BAR frame to the reception STA of the ACKindication information may be transmitting the BAR frame through channelcontention after recognizing that the ACK frame was not received andafter a short interframe space (SIFS), or after a backoff procedure forre-transmitting the data to the reception STA.

When a request for transmission of the BAR frame is received from thereception STA and the BAR frame is transmitted to the reception STA inresponse to the request, the request for the BAR frame may be receivedfrom the reception STA during a random access interval.

When the data field includes an A-MPDU, the A-MPDU may include the atleast one MPDU.

According to another aspect of the present invention, there is providedan access point (AP) device in a wireless local area network (WLAN)system, including: a radio frequency (RF) unit configured to transmitand receive a wireless signal; and a processor configured to control theRF unit, wherein the processor is further configured to generate adownlink (DL) multi-user (MU) physical protocol data unit (PPDU)including a physical preamble and a data field and transmits the DL MUPPDU, wherein the data field includes at least one MAC protocol dataunit (MPDU), the at least one MPDU includes a MAC header and a MAC framebody, and the MAC header includes ACK indication information for uplink(UL) MU transmission of an ACK frame as a response to data transmittedthrough the data field, and transmits the DL MU PPDU.

The at least one MPDU may include an indicator indicating that the atleast one MPDU includes the ACK indication information.

Advantageous Effects

According to an embodiment of the present invention, the AP device mayDL MU-transmit a MAC header including ACK indication informationindicating a UL MU resource for transmitting an ACK frame, and areception STA may transmit an ACK frame using a UL MU resource indicatedby the received ACK indication information.

Also, according to an embodiment of the present invention, since the APdevice may DL MU-transmit an indicator indicating whether the ACKindication information is included, together with the ACK indicationinformation, the reception STA may recognize whether the ACK indicationinformation is included through the indicator.

Other advantages and effects of the present invention will be furtherdescribed in the following embodiments.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichthe present invention may be applied;

FIG. 2 is a diagram illustrating the structure of a layer architectureof an IEEE 802.11 system to which the present invention may be applied;

FIG. 3 illustrates a non-HT format PPDU and an HT format PPDU in awireless communication system to which the present invention may beapplied;

FIG. 4 illustrates a VHT format PPDU in a wireless communication systemto which the present invention may be applied;

FIG. 5 illustrates constellation diagrams for classifying a PPDU formatin a wireless communication system to which the present invention may beapplied;

FIG. 6 illustrates a MAC frame format in an IEEE 802.11 system to whichthe present invention may be applied;

FIG. 7 is a diagram illustrating the frame control field in the MACframe in a wireless communication system to which the present inventionmay be applied;

FIG. 8 illustrates the VHT format of an HT control field in a wirelesscommunication system to which the present invention may be applied;

FIG. 9 is a diagram illustrating a random backoff period and a frametransmission procedure in a wireless communication system to which thepresent invention may be applied;

FIG. 10 is a diagram illustrating an IFS relation in a wirelesscommunication system to which the present invention may be applied;

FIG. 11 is a diagram conceptually showing a method of channel soundingin a wireless communication system to which the present invention may beapplied;

FIG. 12 is a diagram illustrating a VHT NDPA frame in a wirelesscommunication system to which the present invention may be applied;

FIG. 13 is a diagram illustrating an NDP PPDU in a wirelesscommunication system to which the present invention may be applied;

FIG. 14 is a diagram illustrating a VHT compressed beamforming frameformat in a wireless communication system to which the present inventionmay be applied;

FIG. 15 is a diagram illustrating a Beamforming Report Poll frame formatin a wireless communication system to which the present invention may beapplied;

FIG. 16 is a diagram illustrating a Group ID Management frame in awireless communication system to which the present invention may beapplied;

FIG. 17 is a diagram illustrating a downlink multi-user PPDU format in awireless communication system to which the present invention may beapplied;

FIG. 18 is a diagram illustrating a downlink multi-user PPDU format in awireless communication system to which the present invention may beapplied;

FIG. 19 is a diagram illustrating a downlink MU-MIMO transmissionprocess in a wireless communication system to which the presentinvention may be applied;

FIG. 20 is a diagram illustrating an ACK frame in a wirelesscommunication system to which the present invention may be applied;

FIG. 21 is a diagram illustrating a Block Ack Request frame in awireless communication system to which the present invention may beapplied;

FIG. 22 is a diagram illustrating the BAR Information field of a BlockAck Request frame in a wireless communication system to which thepresent invention may be applied;

FIG. 23 is a diagram illustrating a Block Ack frame in a wirelesscommunication system to which the present invention may be applied;

FIG. 24 is a diagram illustrating the BA Information field of a BlockAck frame in a wireless communication system to which the presentinvention may be applied;

FIG. 25 is a diagram illustrating a high efficiency (HE) format PPDUaccording to an embodiment of the present invention;

FIGS. 26 to 28 are diagrams illustrating a HE format PPDU according toan embodiment of the present invention.

FIG. 29 is a diagram illustrating an uplink multi-user transmissionprocedure according to an embodiment of the present invention.

FIGS. 30 to 32 are diagrams illustrating a resource allocation unit inan OFDMA multi-user transmission method according to an embodiment ofthe present invention.

FIG. 33 is a diagram illustrating an embodiment of a 20 MHz DL MU PPDUin which ACK indication information is included in a physical preamble.

FIG. 34 is a diagram illustrating an embodiment of 20 MHz DL MU PPDU inwhich ACK indication information is included in a data field.

FIG. 35 is a diagram illustrating a control field of an HT format.

FIG. 36 is a diagram illustrating an HE control field according to anembodiment of the present invention.

FIG. 37 is a diagram schematically illustrating an error recoveryprocedure according to an embodiment of the present invention.

FIG. 38 is a flow chart illustrating a DL MU transmission method of anAP device according to an embodiment of the present invention.

FIG. 39 is a block diagram of each STA device according to an embodimentof the present invention.

BEST MODES

Reference will now be made in detail to the exemplary embodiments of thepresent invention with reference to the accompanying drawings. Thedetailed description, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present invention, rather than to show the only embodiments that maybe implemented according to the invention. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

The following technologies may be used in a variety of wirelesscommunication systems, such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and non-orthogonalmultiple access (NOMA). CDMA may be implemented using a radiotechnology, such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be implemented using a radio technology, such asglobal system for Mobile communications (GSM)/general packet radioservice (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMA maybe implemented using a radio technology, such as institute of electricaland electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX),IEEE 802.20, or evolved UTRA (E-UTRA). UTRA is part of a universalmobile telecommunications system (UMTS). 3rd generation partnershipproject (3GPP) long term evolution (LTE) is part of an evolved UMTS(E-UMTS) using evolved UMTS terrestrial radio access (E-UTRA), and itadopts OFDMA in downlink and adopts SC-FDMA in uplink. LTE-advanced(LTE-A) is the evolution of 3GPP LTE.

Embodiments of the present invention may be supported by the standarddocuments disclosed in at least one of IEEE 802, 3GPP, and 3GPP2, thatis, radio access systems. That is, steps or portions that belong to theembodiments of the present invention and that are not described in orderto clearly expose the technical spirit of the present invention may besupported by the documents. Furthermore, all terms disclosed in thisdocument may be described by the standard documents.

In order to more clarify a description, IEEE 802.11 system is chieflydescribed, but the technical characteristics of the present inventionare not limited thereto.

General System

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichan embodiment of the present invention may be applied.

The IEEE 802.11 configuration may include a plurality of elements. Theremay be provided a wireless communication system supporting transparentstation (STA) mobility for a higher layer through an interaction betweenthe elements. A basic service set (BSS) may correspond to a basicconfiguration block in an IEEE 802.11 system.

FIG. 1 illustrates that three BSSs BSS 1 to BSS 3 are present and twoSTAs (e.g., an STA 1 and an STA 2 are included in the BSS 1, an STA 3and an STA 4 are included in the BSS 2, and an STA 5 and an STA 6 areincluded in the BSS 3) are included as the members of each BSS.

In FIG. 1, an ellipse indicative of a BSS may be interpreted as beingindicative of a coverage area in which STAs included in thecorresponding BSS maintain communication. Such an area may be called abasic service area (BSA). When an STA moves outside the BSA, it isunable to directly communicate with other STAs within the correspondingBSA.

In the IEEE 802.11 system, the most basic type of a BSS is anindependent a BSS (IBSS). For example, an IBSS may have a minimum formincluding only two STAs. Furthermore, the BSS 3 of FIG. 1 which is thesimplest form and from which other elements have been omitted maycorrespond to a representative example of the IBSS. Such a configurationmay be possible if STAs can directly communicate with each other.Furthermore, a LAN of such a form is not previously planned andconfigured, but may be configured when it is necessary. This may also becalled an ad-hoc network.

When an STA is powered off or on or an STA enters into or exits from aBSS area, the membership of the STA in the BSS may be dynamicallychanged. In order to become a member of a BSS, an STA may join the BSSusing a synchronization process. In order to access all of services in aBSS-based configuration, an STA needs to be associated with the BSS.Such association may be dynamically configured, and may include the useof a distribution system service (DSS).

In an 802.11 system, the distance of a direct STA-to-STA may beconstrained by physical layer (PHY) performance. In any case, the limitof such a distance may be sufficient, but communication between STAs ina longer distance may be required, if necessary. In order to supportextended coverage, a distribution system (DS) may be configured.

The DS means a configuration in which BSSs are interconnected. Morespecifically, a BSS may be present as an element of an extended form ofa network including a plurality of BSSs instead of an independent BSS asin FIG. 1.

The DS is a logical concept and may be specified by the characteristicsof a distribution system medium (DSM). In the IEEE 802.11 standard, awireless medium (WM) and a distribution system medium (DSM) arelogically divided. Each logical medium is used for a different purposeand used by a different element. In the definition of the IEEE 802.11standard, such media are not limited to the same one and are also notlimited to different ones. The flexibility of the configuration (i.e., aDS configuration or another network configuration) of an IEEE 802.11system may be described in that a plurality of media is logicallydifferent as described above. That is, an IEEE 802.11 systemconfiguration may be implemented in various ways, and a correspondingsystem configuration may be independently specified by the physicalcharacteristics of each implementation example.

The DS can support a mobile device by providing the seamless integrationof a plurality of BSSs and providing logical services required to handlean address to a destination.

An AP means an entity which enables access to a DS through a WM withrespect to associated STAs and has the STA functionality. The movementof data between a BSS and the DS can be performed through an AP. Forexample, each of the STA 2 and the STA 3 of FIG. 1 has the functionalityof an STA and provides a function which enables associated STAs (e.g.,the STA 1 and the STA 4) to access the DS. Furthermore, all of APsbasically correspond to an STA, and thus all of the APs are entitiescapable of being addressed. An address used by an AP for communicationon a WM and an address used by an AP for communication on a DSM may notneed to be necessarily the same.

Data transmitted from one of STAs, associated with an AP, to the STAaddress of the AP may be always received by an uncontrolled port andprocessed by an IEEE 802.1X port access entity. Furthermore, when acontrolled port is authenticated, transmission data (or frame) may bedelivered to a DS.

A wireless network having an arbitrary size and complexity may include aDS and BSSs. In an IEEE 802.11 system, a network of such a method iscalled an extended service set (ESS) network. The ESS may correspond toa set of BSSs connected to a single DS. However, the ESS does notinclude a DS. The ESS network is characterized in that it looks like anIBSS network in a logical link control (LLC) layer. STAs included in theESS may communicate with each other. Mobile STAs may move from one BSSto the other BSS (within the same ESS) in a manner transparent to theLLC layer.

In an IEEE 802.11 system, the relative physical positions of BSSs inFIG. 1 are not assumed, and the following forms are all possible.

More specifically, BSSs may partially overlap, which is a form commonlyused to provide consecutive coverage. Furthermore, BSSs may not bephysically connected, and logically there is no limit to the distancebetween BSSs.

Furthermore, BSSs may be placed in the same position physically and maybe used to provide redundancy. Furthermore, one (or one or more) IBSS orESS networks may be physically present in the same space as one or moreESS networks. This may correspond to an ESS network form if an ad-hocnetwork operates at the position in which an ESS network is present, ifIEEE 802.11 networks that physically overlap are configured by differentorganizations, or if two or more different access and security policiesare required at the same position.

In a WLAN system, an STA is an apparatus operating in accordance withthe medium access control (MAC)/PHY regulations of IEEE 802.11. An STAmay include an AP STA and a non-AP STA unless the functionality of theSTA is not individually different from that of an AP. In this case,assuming that communication is performed between an STA and an AP, theSTA may be interpreted as being a non-AP STA. In the example of FIG. 1,the STA 1, the STA 4, the STA 5, and the STA 6 correspond to non-APSTAs, and the STA 2 and the STA 3 correspond to AP STAs.

A non-AP STA corresponds to an apparatus directly handled by a user,such as a laptop computer or a mobile phone. In the followingdescription, a non-AP STA may also be called a wireless device, aterminal, user equipment (UE), a mobile station (MS), a mobile terminal,a wireless terminal, a wireless transmit/receive unit (WTRU), a networkinterface device, a machine-type communication (MTC) device, amachine-to-machine (M2M) device or the like.

Furthermore, an AP is a concept corresponding to a base station (BS), anode-B, an evolved Node-B (eNB), a base transceiver system (BTS), afemto BS or the like in other wireless communication fields.

Hereinafter, in this specification, downlink (DL) means communicationfrom an AP to a non-AP STA. Uplink (UL) means communication from anon-AP STA to an AP. In DL, a transmitter may be part of an AP, and areceiver may be part of a non-AP STA. In UL, a transmitter may be partof a non-AP STA, and a receiver may be part of an AP.

FIG. 2 is a diagram illustrating the structure of a layer architectureof an IEEE 802.11 system to which an embodiment of the present inventionmay be applied.

Referring to FIG. 2, the layer architecture of the IEEE 802.11 systemmay include an MAC sublayer and a PHY sublayer.

The PHY sublayer may be divided into a physical layer convergenceprocedure (PLCP) entity and a physical medium dependent (PMD) entity. Inthis case, the PLCP entity functions to connect the MAC sublayer and adata frame, and the PMD entity functions to wirelessly transmit andreceive data to and from two or more STAs.

The MAC sublayer and the PHY sublayer may include respective managemententities, which may be referred to as an MAC sublayer management entity(MLME) and a PHY sublayer management entity (PLME), respectively. Themanagement entities provide a layer management service interface throughthe operation of a layer management function. The MLME is connected tothe PLME and may perform the management operation of the MAC sublayer.Likewise, the PLME is also connected to the MLME and may perform themanagement operation of the PHY sublayer.

In order to provide a precise MAC operation, a station management entity(SME) may be present in each STA. The SME is a management entityindependent of each layer, and collects layer-based state informationfrom the MLME and the PLME or sets the values of layer-specificparameters. The SME may perform such a function instead of common systemmanagement entities and may implement a standard management protocol.

The MLME, the PLME, and the SME may interact with each other usingvarious methods based on primitives. More specifically, anXX-GET.request primitive is used to request the value of a managementinformation base (MIB) attribute. An XX-GET.confirm primitive returnsthe value of a corresponding MIB attribute if the state is “SUCCESS”,and indicates an error in the state field and returns the value in othercases. An XX-SET.request primitive is used to make a request so that adesignated MIB attribute is set as a given value. If an MIB attributemeans a specific operation, such a request requests the execution of thespecific operation. Furthermore, an XX-SET.confirm primitive means thata designated MIB attribute has been set as a requested value if thestate is “SUCCESS.” In other cases, the XX-SET.confirm primitiveindicates that the state field is an error situation. If an MIBattribute means a specific operation, the primitive may confirm that acorresponding operation has been performed.

An operation in each sublayer is described in brief as follows.

The MAC sublayer generates one or more MAC protocol data units (MPDUs)by attaching an MAC header and a frame check sequence (FCS) to a MACservice data unit (MSDU) received from a higher layer (e.g., an LLClayer) or the fragment of the MSDU. The generated MPDU is delivered tothe PHY sublayer.

If an aggregated MSDU (A-MSDU) scheme is used, a plurality of MSDUs maybe aggregated into a single aggregated MSDU (A-MSDU). The MSDUaggregation operation may be performed in an MAC higher layer. TheA-MSDU is delivered to the PHY sublayer as a single MPDU (if it is notfragmented).

The PHY sublayer generates a physical protocol data unit (PPDU) byattaching an additional field, including information for a PHYtransceiver, to a physical service data unit (PSDU) received from theMAC sublayer. The PPDU is transmitted through a wireless medium.

The PSDU has been received by the PHY sublayer from the MAC sublayer,and the MPDU has been transmitted from the MAC sublayer to the PHYsublayer. Accordingly, the PSDU is substantially the same as the MPDU.

If an aggregated MPDU (A-MPDU) scheme is used, a plurality of MPDUs (inthis case, each MPDU may carry an A-MSDU) may be aggregated in a singleA-MPDU. The MPDU aggregation operation may be performed in an MAC lowerlayer. The A-MPDU may include an aggregation of various types of MPDUs(e.g., QoS data, acknowledge (ACK), and a block ACK (BlockAck)). The PHYsublayer receives an A-MPDU, that is, a single PSDU, from the MACsublayer. That is, the PSDU includes a plurality of MPDUs. Accordingly,the A-MPDU is transmitted through a wireless medium within a singlePPDU.

Physical Protocol Data Unit (PPDU) Format

A PPDU means a data block generated in the physical layer. A PPDU formatis described below based on an IEEE 802.11 a WLAN system to which anembodiment of the present invention may be applied.

FIG. 3 illustrates a non-HT format PPDU and an HT format PPDU in awireless communication system to which an embodiment of the presentinvention may be applied.

FIG. 3(a) illustrates a non-HT format PPDU for supporting IEEE 802.11a/gsystems. The non-HT PPDU may also be called a legacy PPDU.

Referring to FIG. 3(a), the non-HT format PPDU is configured to includea legacy format preamble, including a legacy (or non-HT) short trainingfield (L-STF), a legacy (or non-HT) long training field (L-LTF), and alegacy (or non-HT) signal (L-SIG) field, and a data field.

The L-STF may include a short training orthogonal frequency divisionmultiplexing symbol (OFDM). The L-STF may be used for frame timingacquisition, automatic gain control (AGC), diversity detection, andcoarse frequency/time synchronization.

The L-LTF may include a long training OFDM symbol. The L-LTF may be usedfor fine frequency/time synchronization and channel estimation.

The L-SIG field may be used to send control information for thedemodulation and decoding of the data field.

The L-SIG field may include a rate field of four bits, a reserved fieldof 1 bit, a length field of 12 bits, a parity bit of 1 bit, and a signaltail field of 6 bits.

The rate field includes transfer rate information, and the length fieldindicates the number of octets of a PSDU.

FIG. 3(b) illustrates an HT mixed format PPDU for supporting both anIEEE 802.11n system and IEEE 802.11a/g system.

Referring to FIG. 3(b), the HT mixed format PPDU is configured toinclude a legacy format preamble including an L-STF, an L-LTF, and anL-SIG field, an HT format preamble including an HT-signal (HT-SIG)field, a HT short training field (HT-STF), and a HT long training field(HT-LTF), and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and are the same as those of the non-HT formatfrom the L-STF to the L-SIG field. An L-STA may interpret a data fieldthrough an L-LTF, an L-LTF, and an L-SIG field although it receives anHT mixed PPDU. In this case, the L-LTF may further include informationfor channel estimation to be performed by an HT-STA in order to receivethe HT mixed PPDU and to demodulate the L-SIG field and the HT-SIGfield.

An HT-STA may be aware of an HT mixed format PPDU using the HT-SIG fieldsubsequent to the legacy fields, and may decode the data field based onthe HT mixed format PPDU.

The HT-LTF may be used for channel estimation for the demodulation ofthe data field. IEEE 802.11n supports single user multi-input andmulti-output (SU-MIMO) and thus may include a plurality of HT-LTFs forchannel estimation with respect to each of data fields transmitted in aplurality of spatial streams.

The HT-LTF may include a data HT-LTF used for channel estimation for aspatial stream and an extension HT-LTF additionally used for fullchannel sounding. Accordingly, a plurality of HT-LTFs may be the same asor greater than the number of transmitted spatial streams.

In the HT mixed format PPDU, the L-STF, the L-LTF, and the L-SIG fieldsare first transmitted so that an L-STA can receive the L-STF, the L-LTF,and the L-SIG fields and obtain data. Thereafter, the HT-SIG field istransmitted for the demodulation and decoding of data transmitted for anHT-STA.

An L-STF, an L-LTF, L-SIG, and HT-SIG fields are transmitted withoutperforming beamforming up to an HT-SIG field so that an L-STA and anHT-STA can receive a corresponding PPDU and obtain data. In an HT-STF,an HT-LTF, and a data field that are subsequently transmitted, radiosignals are transmitted through precoding. In this case, an HT-STF istransmitted so that an STA receiving a corresponding PPDU by performingprecoding may take into considerate a portion whose power is varied byprecoding, and a plurality of HT-LTFs and a data field are subsequentlytransmitted.

Table 1 below illustrates the HT-SIG field.

TABLE 1 Field Bit Description MCS 7 Indicate a modulation and codingscheme CBW 20/40 1 Set to “0” if a CBW is 20 MHz or 40 MHz orupper/lower Set to “1” if a CBW is 40 MHz HT length 16 Indicate thenumber of data octets within a PSDU Smoothing 1 Set to “1” if channelsmoothing is recommended Set to “0” if channel estimation is recommendedunsmoothingly for each carrier Not-sounding 1 Set to “0” if a PPDU is asounding PPDU Set to “1” if a PPDU is not a sounding PPDU Reserved 1 Setto “1” Aggregation 1 Set to “1” if a PPDU includes an A-MPDU Set to “0”if not Space-time 2 Indicate a difference between the number ofspace-time block coding streams (NSTS) and the number of spatial streams(NSS) (STBC) indicated by an MCS Set to “00” if an STBC is not used FECcoding 1 Set to “1” if low-density parity check (LDPC) is used Set to“0” if binary convolutional code (BCC) is used Short GI 1 Set to “1” ifa short guard interval (GI) is used after HT training Set to “0” if notNumber of 2 Indicate the number of extension spatial streams (NESSs)extension Set to “0” if there is no NESS spatial Set to “1” if thenumber of NESSs is 1 streams Set to “2” if the number of NESSs is 2 Setto “3” if the number of NESSs is 3 CRC 8 Include CRS for detecting anerror of a PPDU on the receiver side Tail bits 6 Used to terminate thetrellis of a convolutional decoder Set to “0”

FIG. 3(c) illustrates an HT-green field format PPDU (HT-GF format PPDU)for supporting only an IEEE 802.11n system.

Referring to FIG. 3(c), the HT-GF format PPDU includes an HT-GF-STF, anHT-LTF1, an HT-SIG field, a plurality of HT-LTF2s, and a data field.

The HT-GF-STF is used for frame timing acquisition and AGC.

The HT-LTF1 is used for channel estimation.

The HT-SIG field is used for the demodulation and decoding of the datafield.

The HT-LTF2 is used for channel estimation for the demodulation of thedata field. Likewise, an HT-STA uses SU-MIMO. Accordingly, a pluralityof the HT-LTF2s may be configured because channel estimation isnecessary for each of data fields transmitted in a plurality of spatialstreams.

The plurality of HT-LTF2s may include a plurality of data HT-LTFs and aplurality of extension HT-LTFs like the HT-LTF of the HT mixed PPDU.

In FIGS. 3(a) to 3(c), the data field is a payload and may include aservice field, a scrambled PSDU (PSDU) field, tail bits, and paddingbits. All of the bits of the data field are scrambled.

FIG. 3(d) illustrates a service field included in the data field. Theservice field has 16 bits. The 16 bits are assigned No. 0 to No. 15 andare sequentially transmitted from the No. 0 bit. The No. 0 bit to theNo. 6 bit are set to 0 and are used to synchronize a descrambler withina reception stage.

An IEEE 802.11ac WLAN system supports the transmission of a DLmulti-user multiple input multiple output (MU-MIMO) method in which aplurality of STAs accesses a channel at the same time in order toefficiently use a radio channel. In accordance with the MU-MIMOtransmission method, an AP may simultaneously transmit a packet to oneor more STAs that have been subjected to MIMO pairing.

Downlink multi-user transmission (DL MU transmission) means a technologyin which an AP transmits a PPDU to a plurality of non-AP STAs throughthe same time resources using one or more antennas.

Hereinafter, an MU PPDU means a PPDU which delivers one or more PSDUsfor one or more STAs using the MU-MIMO technology or the OFDMAtechnology. Furthermore, an SU PPDU means a PPDU having a format inwhich only one PSDU can be delivered or which does not have a PSDU.

For MU-MIMO transmission, the size of control information transmitted toan STA may be relatively larger than the size of 802.11n controlinformation. Control information additionally required to supportMU-MIMO may include information indicating the number of spatial streamsreceived by each STA and information related to the modulation andcoding of data transmitted to each STA may correspond to the controlinformation, for example.

Accordingly, when MU-MIMO transmission is performed to provide aplurality of STAs with a data service at the same time, the size oftransmitted control information may be increased according to the numberof STAs which receive the control information.

In order to efficiently transmit the control information whose size isincreased as described above, a plurality of pieces of controlinformation required for MU-MIMO transmission may be divided into twotypes of control information: common control information that isrequired for all of STAs in common and dedicated control informationindividually required for a specific STA, and may be transmitted.

FIG. 4 illustrates a VHT format PPDU in a wireless communication systemto which an embodiment of the present invention may be applied.

FIG. 4(a) illustrates a VHT format PPDU for supporting an IEEE 802.11acsystem.

Referring to FIG. 4(a), the VHT format PPDU is configured to include alegacy format preamble including an L-STF, an L-LTF, and an L-SIG field,a VHT format preamble including a VHT-signal-A (VHT-SIG-A) field, a VHTshort training field (VHT-STF), a VHT long training field (VHT-LTF), anda VHT-signal-B (VHT-SIG-B) field, and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and have the same formats as those of the non-HTformat. In this case, the L-LTF may further include information forchannel estimation which will be performed in order to demodulate theL-SIG field and the VHT-SIG-A field.

The L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-A field may berepeated in a 20 MHz channel unit and transmitted. For example, when aPPDU is transmitted through four 20 MHz channels (i.e., an 80 MHzbandwidth), the L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-Afield may be repeated every 20 MHz channel and transmitted.

A VHT-STA may be aware of the VHT format PPDU using the VHT-SIG-A fieldsubsequent to the legacy fields, and may decode the data field based onthe VHT-SIG-A field.

In the VHT format PPDU, the L-STF, the L-LTF, and the L-SIG field arefirst transmitted so that even an L-STA can receive the VHT format PPDUand obtain data. Thereafter, the VHT-SIG-A field is transmitted for thedemodulation and decoding of data transmitted for a VHT-STA.

The VHT-SIG-A field is a field for the transmission of controlinformation that is common to a VHT STAs that are MIMO-paired with anAP, and includes control information for interpreting the received VHTformat PPDU.

The VHT-SIG-A field may include a VHT-SIG-A1 field and a VHT-SIG-A2field.

The VHT-SIG-A1 field may include information about a channel bandwidth(BW) used, information about whether space time block coding (STBC) isapplied or not, a group identifier (ID) for indicating a group ofgrouped STAs in MU-MIMO, information about the number of streams used(the number of space-time streams (NSTS)/part association identifier(AID), and transmit power save forbidden information. In this case, thegroup ID means an identifier assigned to a target transmission STA groupin order to support MU-MIMO transmission, and may indicate whether thepresent MIMO transmission method is MU-MIMO or SU-MIMO.

Table 2 illustrates the VHT-SIG-A1 field.

TABLE 2 field bit description BW 2 Set to “0” if a BW is 20 MHz Set to“1” if a BW is 40 MHz Set to “2” if a BW is 80 MHz Set to “3” if a BW is160 MHz or 80 + 80 MHz Reserved 1 STBC 1 In the case of a VHT SU PPDU:Set to “1” if STBC is used Set to “0” if not In the case of a VHT MUPPDU: Set to “0” group ID 6 Indicate a group ID “0” or “63” indicates aVHT SU PPDU, but indicates a VHT MU PPDU if not NSTS/Partial 12 In thecase of a VHT MU PPDU, divide into 4 user AID positions “p” each havingthree bits “0” if a space-time stream is 0 “1” if a space-time stream is1 “2” if a space-time stream is 2 “3” if a space-time stream is 3 “4” ifa space-time stream is 4 In the case of a VHT SU PPDU, Upper 3 bits areset as follows: “0” if a space-time stream is 1 “1” if a space-timestream is 2 “2” if a space-time stream is 3 “3” if a space-time streamis 4 “4” if a space-time stream is 5 “5” if a space-time stream is 6 “6”if a space-time stream is 7 “7” if a space-time stream is 8 Lower 9 bitsindicate a partial AID. TXOP_PS_NOT_ALLOWED 1 Set to “0” if a VHT APpermits a non-AP VHT STA to switch to power save mode duringtransmission opportunity (TXOP) Set to “1” if not In the case of a VHTPPDU transmitted by a non-AP VHT STA Set to “1” Reserved 1

The VHT-SIG-A2 field may include information about whether a short guardinterval (GI) is used or not, forward error correction (FEC)information, information about a modulation and coding scheme (MCS) fora single user, information about the type of channel coding for multipleusers, beamforming-related information, redundancy bits for cyclicredundancy checking (CRC), the tail bits of a convolutional decoder andso on.

Table 3 illustrates the VHT-SIG-A2 field.

TABLE 3 field bit description Short GI 1 Set to “0” if a short GI is notused in a data field Set to “1” if a short GI is used in a data fieldShort GI 1 Set to “1” if a short GI is used and an extra symbol isdisam- required for the payload of a PPDU biguation Set to “0” if anextra symbol is not required SU/MU 1 In the case of a VHT SU PPDU:coding Set to “0” in the case of binary convolutional code (BCC) Set to“1” in the case of low-density parity check (LDPC) In the case of a VHTMU PPDU: Indicate coding used if the NSTS field of a user whose userposition is “0” is not “0” Set to “0” in the case of BCC Set to “1” inthe case of PDPC Set to “1” as a reserved field if the NSTS field of auser whose user position is “0” is “0” LDPC Extra 1 Set to “1” if anextra OFDM symbol is required due to OFDM an PDPC PPDU encodingprocedure (in the case of a symbol SU PPDU) or the PPDU encodingprocedure of at least one PDPC user (in the case of a VHT MU PPDU) Setto “0” if not SU VHT 4 In the case of a VHT SU PPDU: MCS/MU Indicate aVHT-MCS index coding In the case of a VHT MU PPDU: Indicate coding foruser positions “1” to “3” sequentially from upper bits Indicate codingused if the NSTS field of each user is not “1” Set to “0” in the case ofBCC Set to “1” in the case of LDPC Set to “1” as a reserved field if theNSTS field of each user is “0” Beamformed 1 In the case of a VHT SUPPDU: Set to “1” if a beamforming steering matrix is applied to SUtransmission Set to “0” if not In the case of a VHT MU PPDU: Set to “1”as a reserved field Reserved 1 CRC 8 Include CRS for detecting an errorof a PPDU on the receiver side Tail 6 Used to terminate the trellis of aconvolutional decoder Set to “0”

The VHT-STF is used to improve AGC estimation performance in MIMOtransmission.

The VHT-LTF is used for a VHT-STA to estimate an MIMO channel. Since aVHT WLAN system supports MU-MIMO, the VHT-LTF may be configured by thenumber of spatial streams through which a PPDU is transmitted.Additionally, if full channel sounding is supported, the number ofVHT-LTFs may be increased.

The VHT-SIG-B field includes dedicated control information which isnecessary for a plurality of MU-MIMO-paired VHT-STAs to receive a PPDUand to obtain data. Accordingly, only when common control informationincluded in the VHT-SIG-A field indicates that a received PPDU is forMU-MIMO transmission, a VHT-STA may be designed to decode the VHT-SIG-Bfield. In contrast, if common control information indicates that areceived PPDU is for a single VHT-STA (including SU-MIMO), an STA may bedesigned to not decode the VHT-SIG-B field.

The VHT-SIG-B field includes a VHT-SIG-B length field, a VHT-MCS field,a reserved field, and a tail field.

The VHT-SIG-B length field indicates the length of an A-MPDU (prior toend-of-frame (EOF) padding). The VHT-MCS field includes informationabout the modulation, encoding, and rate-matching of each VHT-STA.

The size of the VHT-SIG-B field may be different depending on the type(MU-MIMO or SU-MIMO) of MIMO transmission and a channel bandwidth usedfor PPDU transmission.

FIG. 4(b) illustrates a VHT-SIG-B field according to a PPDU transmissionbandwidth.

Referring to FIG. 4(b), in 40 MHz transmission, VHT-SIG-B bits arerepeated twice. In 80 MHz transmission, VHT-SIG-B bits are repeated fourtimes, and padding bits set to 0 are attached.

In 160 MHz transmission and 80+80 MHz transmission, first, VHT-SIG-Bbits are repeated four times as in the 80 MHz transmission, and paddingbits set to 0 are attached. Furthermore, a total of the 117 bits isrepeated again.

In a system supporting MU-MIMO, in order to transmit PPDUs having thesame size to STAs paired with an AP, information indicating the size ofthe bits of a data field forming the PPDU and/or information indicatingthe size of bit streams forming a specific field may be included in theVHT-SIG-A field.

In this case, an L-SIG field may be used to effectively use a PPDUformat. A length field and a rate field which are included in the L-SIGfield and transmitted so that PPDUs having the same size are transmittedto all of STAs may be used to provide required information. In thiscase, additional padding may be required in the physical layer becausean MAC protocol data unit (MPDU) and/or an aggregate MAC PDU (A-MPDU)are set based on the bytes (or octets) of the MAC layer.

In FIG. 4, the data field is a payload and may include a service field,a scrambled PSDU, tail bits, and padding bits.

An STA needs to determine the format of a received PPDU because severalformats of PPDUs are mixed and used as described above.

In this case, the meaning that a PPDU (or a PPDU format) is determinedmay be various. For example, the meaning that a PPDU is determined mayinclude determining whether a received PPDU is a PPDU capable of beingdecoded (or interpreted) by an STA. Furthermore, the meaning that a PPDUis determined may include determining whether a received PPDU is a PPDUcapable of being supported by an STA. Furthermore, the meaning that aPPDU is determined may include determining that information transmittedthrough a received PPDU is which information.

This will be described in more detail below with reference to thedrawings.

FIG. 5 illustrates constellation diagrams for classifying a PPDU formatin a wireless communication system to which the present invention may beapplied.

(a) of FIG. 5 illustrates a constellation for the L-SIG field includedin the non-HT format PPDU, (b) of FIG. 5 illustrates a phase rotationfor HT-mixed format PPDU detection, and (c) of FIG. 5 illustrates aphase rotation for VHT format PPDU detection.

In order for an STA to classify a PPDU as a non-HT format PPDU, HT-GFformat PPDU, HT-mixed format PPDU, or VHT format PPDU, the phases ofconstellations of the L-SIG field and of the OFDM symbols, which aretransmitted following the L-SIG field, are used. That is, the STA mayclassify a PDDU format based on the phases of constellations of theL-SIG field of a received PPDU and/or of the OFDM symbols, which aretransmitted following the L-SIG field.

Referring to (a) of FIG. 5, the OFDM symbols of the L-SIG field use BPSK(Binary Phase Shift Keying).

To begin with, in order to classify a PPDU as an HT-GF format PPDU, theSTA, upon detecting a first SIG field from a received PPDU, determineswhether this first SIG field is an L-SIG field or not. That is, the STAattempts to perform decoding based on the constellation illustrated in(a) of FIG. 5. If the STA fails in decoding, the corresponding PPDU maybe classified as the HT-GF format PPDU.

Next, in order to distinguish the non-HT format PPDU, HT-mixed formatPPDU, and VHT format PPDU, the phases of constellations of the OFDMsymbols transmitted following the L-SIG field may be used. That is, themethod of modulation of the OFDM symbols transmitted following the L-SIGfield may vary, and the STA may classify a PPDU format based on themethod of modulation of fields coming after the L-SIG field of thereceived PPDU.

Referring to (b) of FIG. 5, in order to classify a PPDU as an HT-mixedformat PPDU, the phases of two OFDM symbols transmitted following theL-SIG field in the HT-mixed format PPDU may be used.

More specifically, both the phases of OFDM symbols #1 and #2corresponding to the HT-SIG field, which is transmitted following theL-SIG field, in the HT-mixed format PPDU are rotated counterclockwise by90 degrees. That is, the OFDM symbols #1 and #2 are modulated by QBPSK(Quadrature Binary Phase Shift Keying). The QBPSK constellation may be aconstellation which is rotated counterclockwise by 90 degrees based onthe BPSK constellation.

An STA attempts to decode the first and second OFDM symbolscorresponding to the HT-SIG field transmitted after the L-SIG field ofthe received PDU, based on the constellations illustrated in (b) of FIG.5. If the STA succeeds in decoding, the corresponding PPDU may beclassified as an HT format PPDU.

Next, in order to distinguish the non-HT format PPDU and the VHT formatPPDU, the phases of constellations of the OFDM symbols transmittedfollowing the L-SIG field may be used.

Referring to (c) of FIG. 5, in order to classify a PPDU as a VHT formatPPDU, the phases of two OFDM symbols transmitted after the L-SIG fieldmay be used in the VHT format PPDU.

More specifically, the phase of the OFDM symbol #1 corresponding to theVHT-SIG-A coming after the L-SIG field in the HT format PPDU is notrotated, but the phase of the OFDM symbol #2 is rotated counterclockwiseby 90 degrees. That is, the OFDM symbol #1 is modulated by BPSK, and theOFDM symbol #2 is modulated by QBPSK.

The STA attempts to decode the first and second OFDM symbolscorresponding to the VHT-SIG field transmitted following the L-SIG fieldof the received PDU, based on the constellations illustrated in (c) ofFIG. 5. If the STA succeeds in decoding, the corresponding PPDU may beclassified as a VHT format PPDU.

On the contrary, If the STA fails in decoding, the corresponding PPDUmay be classified as a non-HT format PPDU.

MAC Frame Format

FIG. 6 illustrates a MAC frame format in an IEEE 802.11 system to whichthe present invention may be applied.

Referring to FIG. 6, the MAC frame (i.e., an MPDU) includes an MACheader, a frame body, and a frame check sequence (FCS).

The MAC Header is defined as an area, including a frame control field, aduration/ID field, an address 1 field, an address 2 field, an address 3field, a sequence control field, an address 4 field, a QoS controlfield, and an HT control field.

The frame control field contains information on the characteristics ofthe MAC frame. A more detailed description of the frame control fieldwill be given later.

The duration/ID field may be implemented to have a different valuedepending on the type and subtype of a corresponding MAC frame.

If the type and subtype of a corresponding MAC frame is a PS-poll framefor a power save (PS) operation, the duration/ID field may be configuredto include the association identifier (AID) of an STA that hastransmitted the frame. In the remaining cases, the duration/ID field maybe configured to have a specific duration value depending on the typeand subtype of a corresponding MAC frame. Furthermore, if a frame is anMPDU included in an aggregate-MPDU (A-MPDU) format, the duration/IDfield included in an MAC header may be configured to have the samevalue.

The address 1 field to the address 4 field are used to indicate a BSSID,a source address (SA), a destination address (DA), a transmittingaddress (TA) indicating the address of a transmitting STA, and areceiving address (RA) indicating the address of a receiving STA.

An address field implemented as a TA field may be set as a bandwidthsignaling TA value. In this case, the TA field may indicate that acorresponding MAC frame includes additional information in a scramblingsequence. The bandwidth signaling TA may be represented as the MACaddress of an STA that sends a corresponding MAC frame, butindividual/group bits included in the MAC address may be set as aspecific value (e.g., “1”).

The sequence control field is configured to include a sequence numberand a fragment number. The sequence number may indicate a sequencenumber assigned to a corresponding MAC frame. The fragment number mayindicate the number of each fragment of a corresponding MAC frame.

The QoS control field includes information related to QoS. The QoScontrol field may be included if it indicates a QoS data frame in asubtype subfield.

The HT control field includes control information related to an HTand/or VHT transmission/reception scheme. The HT control field isincluded in a control wrapper frame. Furthermore, the HT control fieldis present in a QoS data frame having an order subfield value of 1 and amanagement frame.

The frame body is defined as an MAC payload. Data to be transmitted in ahigher layer is placed in the frame body. The frame body has a varyingsize. For example, a maximum size of an MPDU may be 11454 octets, and amaximum size of a PPDU may be 5.484 ms.

The FCS is defined as an MAC footer and used for the error search of anMAC frame.

The first three fields (i.e., the frame control field, the duration/IDfield, and Address 1 field) and the last field (i.e., the FCS field)form a minimum frame format and are present in all of frames. Theremaining fields may be present only in a specific frame type.

FIG. 7 is a diagram illustrating the frame control field in the MACframe in a wireless communication system to which the present inventionmay be applied.

Referring to FIG. 7, the frame control field includes a Protocol Versionsubfield, a Type subfield, a Subtype subfield, a to DS subfield, a FromDS subfield, a More Fragments subfield, a Retry subfield, a PowerManagement subfield, a More Data subfield, a Protected Frame subfield,and an Order subfield.

The protocol version subfield may indicate the version of a WLANprotocol applied to the MAC frame.

The type subfield and the subtype subfield may be configured to indicateinformation for identifying the function of the MAC frame.

The MAC frame may include three frame types: Management frames, Controlframes, and Data frames.

Each frame type may be subdivided into subtypes.

For example, the Control frames may include an RTS (request-to-send)frame, a CTS (clear-to-send) frame, an ACK (Acknowledgement) frame, aPS-Poll frame, a CF (contention free)-End frame, a CF-End+CF-ACK frame,a BAR (Block Acknowledgement request) frame, a BA (BlockAcknowledgement) frame, a Control Wrapper (Control+HTcontrol) frame, aVHT NDPA (Null Data Packet Announcement) frame, and a Beamforming ReportPoll frame.

The Management frames may include a Beacon frame, an ATIM (AnnouncementTraffic Indication Message) frame, a Disassociation frame, anAssociation Request/Response frame, a Reassociation Request/Responseframe, a Probe Request/Response frame, an Authentication frame, aDeauthentication frame, an Action frame, an Action No ACK frame, and aTiming Advertisement frame.

The To Ds subfield and the From DS subfield may contain informationrequired to interpret the Address 1 field through Address 4 fieldincluded in the MAC frame header. For a Control frame, the To DSsubfield and the From DS subfield may all set to ‘0’. For a Managementframe, the To DS subfield and the From DS subfield may be set to ‘1’ and‘0’, respectively, if the corresponding frame is a QoS Management frame(QMF); otherwise, the To DS subfield and the From DS subfield all may beset to ‘0’.

The More Fragments subfield may indicate whether there is a fragment tobe sent subsequent to the MAC frame. If there is another fragment of thecurrent MSDU or MMPDU, the More Fragments subfield may be set to ‘1’;otherwise, it may be set to ‘0’.

The Retry subfield may indicate whether the MAC frame is the previousMAC frame that is re-transmitted. If the MAC frame is the previous MACframe that is re-transmitted, the Retry subfield may be set to ‘1’;otherwise, it may be set to ‘0’.

The Power Management subfield may indicate the power management mode ofthe STA. If the Power Management subfield has a value of ‘1’, this mayindicate that the STA switches to power save mode.

The More Data subfield may indicate whether there is a MAC frame to beadditionally sent. If there is a MAC frame to be additionally sent, theMore Data subfield may be set to ‘1’; otherwise, it may be set to ‘0’.

The Protected Frame subfield may indicate whether a Frame Body field isencrypted or not. If the Frame Body field contains information that isprocessed by a cryptographic encapsulation algorithm, it may be set to‘1’; otherwise ‘0’.

Information contained in the above-described fields may be as defined inthe IEEE 802.11 system. Also, the above-described fields are examples ofthe fields that may be included in the MAC frame but not limited tothem. That is, the above-described fields may be substituted with otherfields or further include additional fields, and not all of the fieldsmay be necessarily included.

FIG. 8 illustrates the VHT format of an HT control field in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

Referring to FIG. 8, the HT control field may include a VHT subfield, anHT control middle subfield, an AC constraint subfield, and a reversedirection grant (RDG)/more PPDU subfield.

The VHT subfield indicates whether the HT control field has the formatof an HT control field for VHT (VHT=1) or has the format of an HTcontrol field for HT (VHT=0). In FIG. 8, it is assumed that the HTcontrol field is an HT control field for VHT (i.e., VHT=1). The HTcontrol field for VHT may be called a VHT control field.

The HT control middle subfield may be implemented to a different formatdepending on the indication of a VHT subfield. The HT control middlesubfield is described in detail later.

The AC constraint subfield indicates whether the mapped access category(AC) of a reverse direction (RD) data frame is constrained to a singleAC.

The RDG/more PPDU subfield may be differently interpreted depending onwhether a corresponding field is transmitted by an RD initiator or an RDresponder.

Assuming that a corresponding field is transmitted by an RD initiator,the RDG/more PPDU subfield is set as “1” if an RDG is present, and theRDG/more PPDU subfield is set as “0” if an RDG is not present. Assumingthat a corresponding field is transmitted by an RD responder, theRDG/more PPDU subfield is set as “1” if a PPDU including thecorresponding subfield is the last frame transmitted by the RDresponder, and the RDG/more PPDU subfield is set as “0” if another PPDUis transmitted.

As described above, the HT control middle subfield may be implemented toa different format depending on the indication of a VHT subfield.

The HT control middle subfield of an HT control field for VHT mayinclude a reserved bit subfield, a modulation and coding scheme (MCS)feedback request (MRQ) subfield, an MRQ sequence identifier(MSI)/space-time block coding (STBC) subfield, an MCS feedback sequenceidentifier (MFSI)/least significant bit (LSB) of group ID (GID-L)subfield, an MCS feedback (MFB) subfield, a most significant Bit (MSB)of group ID (GID-H) subfield, a coding type subfield, a feedbacktransmission type (FB Tx type) subfield, and an unsolicited MFBsubfield.

Table 4 illustrates a description of each subfield included in the HTcontrol middle subfield of the VHT format.

TABLE 4 subfield meaning definition MRQ MCS request Set to “1” if MCSfeedback (solicited MFB) is not requested Set to “0” if not MSI MRQ AnMSI subfield includes a sequence number sequence within a range of 0 to6 to identify a specific identifier request if an unsolicited MFBsubfield is set to “0” and an MRQ subfield is set to “1.” Include acompressed MSI subfield (2 bits) and an STBC indication subfield (1 bit)if an unsolicited MFB subfield is “1.” MFSI/ MFB An MFSI/GID-L subfieldincludes the received GID-L sequence value of an MSI included within aframe related identifier/ to MFB information if an unsolicited MFB LSBof subfield is set to “0.” group ID An MFSI/GID-L subfield includes thelowest three bits of a group ID of a PPDU estimated by an MFB if an MFBis estimated from an MU PPDU. MFB VHT An MFB subfield includesrecommended MFB. N_STS, VHT-MCS = 15, NUM_STS = 7 indicates that MCS,BW, feedback is not present. SNR feedback GID-H MSB of A GID-H subfieldincludes the most significant group ID bit 3 bits of a group ID of aPPDU whose solicited MFB has been estimated if an unsolicited MFB fieldis set to “1” and MFB has been estimated from a VHT MU PPDU. All ofGID-H subfields are set to “1” if MFB is estimated from an SU PPDU.Coding Coding type If an unsolicited MFB subfield is set to “1”, a Typeor MFB coding type subfield includes the coding type response (binaryconvolutional code (BCC) includes 0 and low-density parity check (LDPC)includes 1) of a frame whose solicited MFB has been estimated FB TxTransmission An FB Tx Type subfield is set to “0” if an Type type of MFBunsolicited MFB subfield is set to “1” and MFB response has beenestimated from an unbeamformed VHT PPDU. An FB Tx Type subfield is setto “1” if an unsolicited MFB subfield is set to “1” and MFB has beenestimated from a beamformed VHT PPDU. Unso- Unsolicited Set to “1” ifMFB is a response to MRQ licited MCS Set to “0” if MFB is not a responseto MRQ MFB feedback indicator

Furthermore, the MFB subfield may include the number of VHT space timestreams (NUM_STS) subfield, a VHT-MCS subfield, a bandwidth (BW)subfield, and a signal to noise ratio (SNR) subfield.

The NUM_STS subfield indicates the number of recommended spatialstreams. The VHT-MCS subfield indicates a recommended MCS. The BWsubfield indicates bandwidth information related to a recommended MCS.The SNR subfield indicates an average SNR value of data subcarriers andspatial streams.

The information included in each of the aforementioned fields may complywith the definition of an IEEE 802.11 system. Furthermore, each of theaforementioned fields corresponds to an example of fields which may beincluded in an MAC frame and is not limited thereto. That is, each ofthe aforementioned fields may be substituted with another field,additional fields may be further included, and all of the fields may notbe essentially included.

Medium Access Mechanism

In IEEE 802.11, communication is basically different from that of awired channel environment because it is performed in a shared wirelessmedium.

In a wired channel environment, communication is possible based oncarrier sense multiple access/collision detection (CSMA/CD). Forexample, when a signal is once transmitted by a transmission stage, itis transmitted up to a reception stage without experiencing great signalattenuation because there is no great change in a channel environment.In this case, when a collision between two or more signals is detected,detection is possible. The reason for this is that power detected by thereception stage becomes instantly higher than power transmitted by thetransmission stage. In a radio channel environment, however, sincevarious factors (e.g., signal attenuation is great depending on thedistance or instant deep fading may be generated) affect a channel, atransmission stage is unable to accurately perform carrier sensingregarding whether a signal has been correctly transmitted by a receptionstage or a collision has been generated.

Accordingly, in a WLAN system according to IEEE 802.11, a carrier sensemultiple access with collision avoidance (CSMA/CA) mechanism has beenintroduced as the basic access mechanism of MAC. The CAMA/CA mechanismis also called a distributed coordination function (DCF) of IEEE 802.11MAC, and basically adopts a “listen before talk” access mechanism. Inaccordance with such a type of access mechanism, an AP and/or an STAperform clear channel assessment (CCA) for sensing a radio channel or amedium for a specific time interval (e.g., a DCF inter-frame space(DIFS)) prior to transmission. If, as a result of the sensing, themedium is determined to be an idle state, the AP and/or the STA startsto transmit a frame through the corresponding medium. In contrast, if,as a result of the sensing, the medium is determined to be a busy state(or an occupied status), the AP and/or the STA do not start theirtransmission, may wait for a delay time (e.g., a random backoff period)for medium access in addition to the DIFS assuming that several STAsalready wait for in order to use the corresponding medium, and may thenattempt frame transmission.

Assuming that several STAs trying to transmit frames are present byapplying the random backoff period, they will wait for different timesbecause the STAs stochastically have different backoff period values andwill attempt frame transmission. In this case, a collision can beminimized by applying the random backoff period.

Furthermore, the IEEE 802.11 MAC protocol provides a hybrid coordinationfunction (HCF). The HCF is based on a DCF and a point coordinationfunction (PCF). The PCF is a polling-based synchronous access method,and refers to a method for periodically performing polling so that allof receiving APs and/or STAs can receive a data frame. Furthermore, theHCF has enhanced distributed channel access (EDCA) and HCF controlledchannel access (HCCA). In EDCA, a provider performs an access method forproviding a data frame to multiple users on a contention basis. In HCCA,a non-contention-based channel access method using a polling mechanismis used. Furthermore, the HCF includes a medium access mechanism forimproving the quality of service (QoS) of a WLAN, and may transmit QoSdata in both a contention period (CP) and a contention-free period(CFP).

FIG. 9 is a diagram illustrating a random backoff period and a frametransmission procedure in a wireless communication system to which anembodiment of the present invention may be applied.

When a specific medium switches from an occupied (or busy) state to anidle state, several STAs may attempt to transmit data (or frames). Inthis case, as a scheme for minimizing a collision, each of the STAs mayselect a random backoff count, may wait for a slot time corresponding tothe selected random backoff count, and may attempt transmission. Therandom backoff count has a pseudo-random integer value and may bedetermined as one of uniformly distributed values in 0 to a contentionwindow (CW) range. In this case, the CW is a CW parameter value. In theCW parameter, CW_min is given as an initial value. If transmission fails(e.g., if ACK for a transmitted frame is not received), the CW_min mayhave a twice value. If the CW parameter becomes CW_max, it may maintainthe CW_max value until data transmission is successful, and the datatransmission may be attempted. If the data transmission is successful,the CW parameter is reset to a CW_min value. The CW, CW_min, and CW_maxvalues may be set to 2̂n−1 (n=0, 1, 2, . . . ,).

When a random backoff process starts, an STA counts down a backoff slotbased on a determined backoff count value and continues to monitor amedium during the countdown. When the medium is monitored as a busystate, the STA stops the countdown and waits. When the medium becomes anidle state, the STA resumes the countdown.

In the example of FIG. 9, when a packet to be transmitted in the MAC ofan STA 3 is reached, the STA 3 may check that a medium is an idle stateby a DIFS and may immediately transmit a frame.

The remaining STAs monitor that the medium is the busy state and wait.In the meantime, data to be transmitted by each of an STA 1, an STA 2,and an STA 5 may be generated. When the medium is monitored as an idlestate, each of the STAs waits for a DIFS and counts down a backoff slotbased on each selected random backoff count value.

The example of FIG. 9 shows that the STA 2 has selected the smallestbackoff count value and the STA 1 has selected the greatest backoffcount value. That is, FIG. 7 illustrates that the remaining backoff timeof the STA 5 is shorter than the remaining backoff time of the STA 1 ata point of time at which the STA 2 finishes a backoff count and startsframe transmission.

The STA 1 and the STA 5 stop countdown and wait while the STA 2 occupiesthe medium. When the occupation of the medium by the STA 2 is finishedand the medium becomes an idle state again, each of the STA 1 and theSTA 5 waits for a DIFS and resumes the stopped backoff count. That is,each of the STA 1 and the STA 5 may start frame transmission aftercounting down the remaining backoff slot corresponding to the remainingbackoff time. The STA 5 starts frame transmission because the STA 5 hasa shorter remaining backoff time than the STA 1.

While the STA 2 occupies the medium, data to be transmitted by an STA 4may be generated. In this case, from a standpoint of the STA 4, when themedium becomes an idle state, the STA 4 waits for a DIFS and counts downa backoff slot corresponding to its selected random backoff count value.

FIG. 9 shows an example in which the remaining backoff time of the STA 5coincides with the random backoff count value of the STA 4. In thiscase, a collision may be generated between the STA 4 and the STA 5. Whena collision is generated, both the STA 4 and the STA 5 do not receiveACK, so data transmission fails. In this case, each of the STA 4 and theSTA 5 doubles its CW value, select a random backoff count value, andcounts down a backoff slot.

The STA 1 waits while the medium is the busy state due to thetransmission of the STA 4 and the STA 5. When the medium becomes an idlestate, the STA 1 may wait for a DIFS and start frame transmission afterthe remaining backoff time elapses.

The CSMA/CA mechanism includes virtual carrier sensing in addition tophysical carrier sensing in which an AP and/or an STA directly sense amedium.

Virtual carrier sensing is for supplementing a problem which may begenerated in terms of medium access, such as a hidden node problem. Forthe virtual carrier sensing, the MAC of a WLAN system uses a networkallocation vector (NAV). The NAV is a value indicated by an AP and/or anSTA which now uses a medium or has the right to use the medium in orderto notify another AP and/or STA of the remaining time until the mediumbecomes an available state. Accordingly, a value set as the NAVcorresponds to the period in which a medium is reserved to be used by anAP and/or an STA that transmit corresponding frames. An STA thatreceives an NAV value is prohibited from accessing the medium during thecorresponding period. The NAV may be set based on the value of theduration field of the MAC header of a frame, for example.

An AP and/or an STA may perform a procedure for exchanging a request tosend (RTS) frame and a clear to send (CTS) frame in order to providenotification that they will access a medium. The RTS frame and the CTSframe include information indicating a temporal section in which awireless medium required to transmit/receive an ACK frame has beenreserved to be accessed if substantial data frame transmission and anacknowledgement response (ACK) are supported. Another STA which hasreceived an RTS frame from an AP and/or an STA attempting to send aframe or which has received a CTS frame transmitted by an STA to which aframe will be transmitted may be configured to not access a mediumduring a temporal section indicated by information included in theRTS/CTS frame. This may be implemented by setting the NAV during a timeinterval.

Interframe Space (IFS)

A time interval between frames is defined as an interframe space (IFS).An STA may determine whether a channel is used during an IFS timeinterval through carrier sensing. In an 802.11 WLAN system, a pluralityof IFSs is defined in order to provide a priority level by which awireless medium is occupied.

FIG. 10 is a diagram illustrating an IFS relation in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

All of pieces of timing may be determined with reference to physicallayer interface primitives, that is, a PHY-TXEND.confirm primitive, aPHYTXSTART.confirm primitive, a PHY-RXSTART.indication primitive, and aPHY-RXEND.indication primitive.

An interframe space (IFS) depending on an IFS type is as follows.

a) A reduced interframe space (IFS) (RIFS)

b) A short interframe space (IFS) (SIFS)

c) A PCF interframe space (IFS) (PIFS)

d) A DCF interframe space (IFS) (DIFS)

e) An arbitration interframe space (IFS) (AIFS)

f) An extended interframe space (IFS) (EIFS)

Different IFSs are determined based on attributes specified by aphysical layer regardless of the bit rate of an STA. IFS timing isdefined as a time gap on a medium. IFS timing other than an AIFS isfixed for each physical layer.

The SIFS is used to transmits a PPDU including an ACK frame, a CTSframe, a block ACK request (BlockAckReq) frame, or a block ACK(BlockAck) frame, that is, an instant response to an A-MPDU, the secondor consecutive MPDU of a fragment burst, and a response from an STA withrespect to polling according to a PCF. The SIFS has the highestpriority. Furthermore, the SIFS may be used for the point coordinator offrames regardless of the type of frame during a non-contention period(CFP) time. The SIFS indicates the time prior to the start of the firstsymbol of the preamble of a next frame which is subsequent to the end ofthe last symbol of a previous frame or from signal extension (ifpresent).

SIFS timing is achieved when the transmission of consecutive frames isstarted in a Tx SIFS slot boundary.

The SIFS is the shortest in IFS between transmissions from differentSTAs. The SIFS may be used if an STA occupying a medium needs tomaintain the occupation of the medium during the period in which theframe exchange sequence is performed.

Other STAs required to wait so that a medium becomes an idle state for alonger gap can be prevented from attempting to use the medium becausethe smallest gap between transmissions within a frame exchange sequenceis used. Accordingly, priority may be assigned in completing a frameexchange sequence that is in progress.

The PIFS is used to obtain priority in accessing a medium.

The PIFS may be used in the following cases.

An STA operating under a PCF

An STA sending a channel switch announcement frame

An STA sending a traffic indication map (TIM) frame

A hybrid coordinator (HC) starting a CFP or transmission opportunity(TXOP)

An HC or non-AP QoS STA, that is, a TXOP holder polled for recoveringfrom the absence of expected reception within a controlled access phase(CAP)

An HT STA using dual CTS protection before sending CTS2

A TXOP holder for continuous transmission after a transmission failure

A reverse direction (RD) initiator for continuous transmission usingerror recovery

An HT AP during a PSMP sequence in which a power save multi-poll (PSMP)recovery frame is transmitted

An HT AT performing CCA within a secondary channel before sending a 40MHz mask PPDU using EDCA channel access

In the illustrated examples, an STA using the PIFS starts transmissionafter a carrier sense (CS) mechanism for determining that a medium is anidle state in a Tx PIFS slot boundary other than the case where CCA isperformed in a secondary channel.

The DIFS may be used by an STA which operates to send a data frame(MPDU) and a MAC management protocol data unit management (MMPDU) frameunder the DCF. An STA using the DCF may transmit data in a TxDIFS slotboundary if a medium is determined to be an idle state through a carriersense (CS) mechanism after an accurately received frame and a backofftime expire. In this case, the accurately received frame means a frameindicating that the PHY-RXEND.indication primitive does not indicate anerror and an FCS indicates that the frame is not an error (i.e., errorfree).

An SIFS time (“aSIFSTime”) and a slot time (“aSlotTime”) may bedetermined for each physical layer. The SIFS time has a fixed value, butthe slot time may be dynamically changed depending on a change in thewireless delay time “aAirPropagationTime.”

The “aSIFSTime” is defined as in Equations 1 and 2 below.

aSIFSTim (16μs)=aRxRFDelay(0.5)+aRxPLCPDelay(12.5)+aMACProcessingDelay(1 or<2)+aRxTxTurnaroundTime(<2)   [Equation 1]

aRxTxTurnaroundTime=aTxPLCPDelay(1)+aRxTxSwitchTime(0.25)+aTxRampOnTime(0.25)+aTxRFDelay(0.5)  [Equation 2]

The “aSlotTime” is defined as in Equation 3 below.

aSlotTime=aCCATime(<4)+aRxTxTurnaroundTime(<2)+aAirPropagationTime(<1)+aMACProcessingDelay(<2)  [Equation 3]

In Equation 3, a default physical layer parameter is based on“aMACProcessingDelay” having a value which is equal to or smaller than 1μs. A radio wave is spread 300 μs in the free space. For example, 3 μsmay be the upper limit of a BSS maximum one-way distance ˜450 m (a roundtrip is ˜900 m).

The PIFS and the SIFS are defined as in Equations 4 and 5, respectively.

PIFS(16 μs)=aSIFSTime+aSlotTime   [Equation 4]

DIFS(34 μ2s)=aSIFSTime+2*aSlotTime   [Equation 5]

In Equations 1 to 5, the numerical value within the parenthesisillustrates a common value, but the value may be different for each STAor for the position of each STA.

The aforementioned SIFS, PIFS, and DIFS are measured based on an MACslot boundary (e.g., a Tx SIFS, a Tx PIFS, and a TxDIFS) different froma medium.

The MAC slot boundaries of the SIFS, the PIFS, and the DIFS are definedas in Equations 6 to 8, respectively.

TxSIFS=SIFS−aRxTxTurnaroundTime   [Equation 6]

TxPIFS=TxSIFS+aSlotTime   [Equation 7]

TxDIFS=TxSIFS+2*aSlotTlme   [Equation 8]

Channel State Information Feedback Method

SU-MIMO technology, in which a beamformer assigns all antennas to onebeamformee for communication, enhances channel capacity throughspatial-temporal diversity gain and multi-stream transmission. SU-MIMOtechnology uses more antennas than when MIMO technology is not used,thereby leveraging spatial degrees of freedom and contributing to theimprovement of a physical layer.

MU-MIMO technology, in which a beamformer assigns antennas to multiplebeamformees, can improve the performance of MIMO antennas by increasingthe per-beamformee transfer rate or channel reliability through a linklayer protocol for multiple access of multiple beamformees connected tothe beamformer.

In MIMO environments, performance depends largely on how accuratechannel information the beamformer acquires. Thus, a feedback procedureis required to acquire channel information.

There are largely two types of feedback supported to acquire channelinformation: one is to use a control frame and the other is to use achannel sounding procedure which does not include a data field. Soundingrefers to using a preamble training field to measure channel for otherpurposes than data demodulation of a PPDU including the correspondingtraining field.

Hereinafter, a channel information feedback method using a control frameand a channel information feedback method using an NDP (null datapacket) will be described in more detail.

1) Feedback using Control Frame

In MIMO environments, a beamformer may instruct a beamformee to sendchannel state information feedback through the HT control field includedin the MAC header, or the beamformee may report channel stateinformation through the HT control field included in the MAC header (seeFIG. 8). The HT control field may be included in a Control Wrapperframe, a QoS Data frame in which the Order subfield of the MAC header isset to 1, and a Management frame.

2) Feedback Using Channel Sounding

FIG. 11 is a diagram conceptually showing a method of channel soundingin a wireless communication system to which the present invention may beapplied.

FIG. 11 illustrates a method of feedback of channel state informationbetween a beamformer (e.g., AP) and a beamformee (e.g., non-AP STA)based on a sounding protocol. The sounding protocol may refer to aprocedure of receiving feedback about information on channel stateinformation.

A method of sounding channel state information between a beamformer anda beamformee based on a sounding protocol may be performed in thefollowing steps:

(1) A beamformer transmits a VHT NDPA (VHT Null Data PacketAnnouncement) frame indicating sounding and transmission for feedbackfrom a beamformee.

The VHT NDPA frame refers to a control frame that is used to indicatethat channel sounding is initiated and an NDP (Null Data Packet) istransmitted. In other words, a VHT NDPA frame may be transmitted beforeNDP transmission to allow a beamformee to ready to feed back channelstate information before receiving the NDP frame.

The VHT NDPA frame may contain AID (association identifier) information,feedback type information, etc. of a beamformee that will transmit anNDP. A more detailed description of the VHT NDPA frame will be givenlater.

The VHT NDPA frame may be transmitted in different ways forMU-MIMO-based data transmission and SU-MIMO-based data transmission. Forexample, in the case of channel sounding for MU-MIMO, the VHT NDPA framemay be transmitted in a broadcast manner, whereas, in the case ofchannel sounding for SU-MIMO, the VHT NDPA frame may be transmitted in aunicast manner.

(2) After transmitting the VHT NDPA frame, the beamformer transmits anNDP after an SIFS. The NDP has a VHT PPDU structure but without a datafield.

Beamformees that have received the VHT NDPA frame may check the value ofthe AID12 subfield included in the STA information field and determinewhether they are a target STA for sounding.

Moreover, the beamformees may know their order of feedback through theSTA Info field included in the NDPA. FIG. 11 illustrates that feedbackoccurs in the order of Beamformee 1, Beamformee 2, and Beamformee 3.

(3) Beamformee 1 acquires downlink channel state information based onthe training field included in the NDP and generates feedbackinformation to send to the beamformer.

Beamformee 1 transmits a VHT compressed beamforming frame containingfeedback information to the beamformer after an SIFS after receiving theNDP frame.

The VHT compressed beamforming frame may include an SNR value for aspace-time stream, information on a compressed beamforming feedbackmatrix for a subcarrier, and so on. A more detailed description of theVHT compressed beamforming frame will be provided later.

(4) The beamformer receives the VHT compressed beamforming frame fromBeamformee 1, and then, after an SIFS, transmits a Beamforming ReportPoll frame to Beamformee 2 in order to acquire channel information fromBeamformee 2.

The Beamforming Report Poll frame is a frame that performs the same roleas the NDP frame. Beamformee 2 may measure channel state based on thetransmitted Beamforming Report Poll frame.

A more detailed description of the Beamforming Report Poll frame will begiven later.

(5) After receiving the Beamforming Report Poll frame, Beamformee 2transmits a VHT Compressed Beamforming frame containing feedbackinformation to the beamformer after an SIFS.

(6) The beamformer receives the VHT Compressed Beamforming frame fromBeamformee 2 and then, after an SIFS, transmits a Beamforming ReportPoll frame to Beamformee 3 in order to acquire channel information fromBeamformee 3.

(7) After receiving the Beamforming Report Poll frame, Beamformee 3transmits a VHT Compressed Beamforming frame containing feedbackinformation to the beamformer after an SIFS.

Hereinafter, a frame used for the above-described channel soundingprocedure will be discussed.

FIG. 12 is a diagram illustrating a VHT NDPA frame in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 12, a VHT NDPA frame may consist of a Frame Controlfield, a Duration field, an RA (Receiving Address) field, a TA(Transmitting Address) field, a Sounding Dialog Token field, an STA Info1 field through STA info n field, and an FCS.

The RA field value indicates the address of a receiver or STA whichreceives the VHT NDPA frame.

If the VHT NDPA frame includes only one STA Info field, then the RAfield is set to the address of the STA identified by the AID in the STAInfo field. For example, when transmitting the VHT NDPA frame to onetarget STA for SU-MIMO channel sounding, an AP unicasts the VHT NDPAframe to the target STA.

On the other hand, if the VHT NDPA frame includes more than one STA Infofield, then the RA field is set to the broadcast address. For example,when transmitting the VHT NDPA frame to at least one target STA forMU-MIMO channel sounding, an AP broadcasts the VHT NDPA frame.

The TA field value indicates the address of a transmitter ortransmitting STA which transmits the VHT NDPA frame or a bandwidthsignaling TA.

The Sounding Dialog Token field also may be called a Sounding Sequencefield. The Sounding Dialog Token Number subfield in the Sounding DialogToken field contains a value selected by the beamformer to identify theVHT NDPA frame.

The VHT NDPA frame includes at least one STA Info field. That is, theVHT NDPA frame includes an STA Info field containing information ontarget STAs for sounding. One STA Info field may be included for eachtarget STA for sounding.

Each STA Info field may include an AID12 subfield, a Feedback Typesubfield, and an NC Index subfield.

Table 5 shows the subfields of an STA Info field included in the VHTNDPA frame.

TABLE 5 Subfield Description AID12 Contains the AID of a target STA forsounding feedback. The AID12 subfield value is set to ‘0’ if the targetSTA is an AP, mesh STA, or STA that is a member of an IBSS. FeedbackIndicates the type of feedback requested for the target STA for Typesounding. Set to 0 for SU-MIMO. Set to 1 for MU-MIMO. Nc Index If theFeedback Type subfield indicates MU-MIMO, then NcIndex indicates thenumber of columns, Nc, in the Compressed Beamforming Feedback Matrixsubfield minus 1. Set to 0 for Nc = 1, Set to 1 for Nc = 2, . . . Set to7 for Nc = 8. Reserved if the Feedback Type subfield indicates SU-MIMO.

Information contained in the above-described fields may be as defined inthe IEEE 802.11 system. Also, the above-described fields are examples ofthe fields that may be included in the MAC frame but not limited tothem. That is, the above-described fields may be substituted with otherfields or further include additional fields.

FIG. 13 is a diagram illustrating an NDP PPDU in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 13, an NDP may have the VHT PPDU format shownpreviously in FIG. 4, but without the data field. The NDP may beprecoded based on a particular precoding matrix and transmitted to atarget STA for sounding.

In the L-SIG field of the NDP, the length field indicating the length ofa PSDU included in the data field is set to ‘0’.

In the VHT-SIG-A field of the NDP, the Group ID field indicating whethera transmission technique used for NDP transmission is MU-MIMO or SU-MIMOis set to a value indicating SU-MIMO transmission.

The data bits of the VHT-SIG-B field of the NDP are set to a fixed bitpattern for each bandwidth.

Upon receiving the NDP, the target STA for sounding performs channelestimation and acquires channel state information.

FIG. 14 is a diagram illustrating a VHT compressed beamforming frameformat in a wireless communication system to which the present inventionmay be applied.

Referring to FIG. 14, the VHT compressed beamforming frame is a VHTAction frame for supporting VHT functionality, and its frame bodyincludes an Action field. The Action field is included in the frame bodyof a MAC frame to provide a mechanism for specifying extended managementactions.

The Action field consists of a Category field, a VHT Action field, a VHTMIMO Control field, a VHT Compressed Beamforming Report field, and an MUExclusive Beamforming Report field.

The Category field is set to a value indicating the VHT category (i.e.,VHT Action frame), and the VHT Action field is set to a value indicatingthe VHT Compressed Beamforming frame.

The VHT MIMO Control field is used to feed back control informationrelated to beamforming feedback. The VHT MIMO Control field may alwaysbe present in the VHT Compressed Beamforming frame.

The VHT Compressed Beamforming Report field is used to feed backinformation on a beamforming matrix containing SNR information forspace-time streams used for transmitting data.

The MU Exclusive Beamforming Report field is used to feed back SNRinformation for spatial streams when performing a MU-MIMO transmission.

The presence and content of the VHT Compressed Beamforming Report fieldand the MU Exclusive Beamforming Report field are dependent on thevalues of the Feedback Type, Remaining Feedback Segments, and FirstFeedback Segment subfields of the VHT MIMO Control field

Hereinafter, the VHT MIMO Control field, the VHT Compressed BeamformingReport field, and the MU Exclusive Beamforming Report field may bediscussed more concretely.

1) The VHT MIMO Control field consists of an Nc index subfield, an NrIndex subfield, a Channel Width subfield, a Grouping subfield, aCodebook Information subfield, a Feedback type subfield, a RemainingFeedback segments subfield, a First Feedback segment subfield, areserved subfield, and a Sounding Dialog Token Number field.

Table 6 shows the subfields of the VHT MIMO Control field.

TABLE 6 Subfield Bits Description Nc Index 3 Indicates the number ofcolumns, Nc, in the compressed beamforming feedback matrix minus 1: Setto 0 for Nc = 1, Set to 1 for Nc = 2, . . . Set to 7 for Nc = 8. NrIndex 3 Indicates the number of rows, Nr, in the compressed beamformingfeedback matrix minus 1: Set to 0 for Nr = 1, Set to 1 for Nr = 2, . . .Set to 7 for Nr = 8. Channel 2 Indicates the width of the channelmeasured to Width create a compressed beamforming feedback matrix: Setto 0 for 20 MHz, Set to 1 for 40 MHz, Set to 2 for 80 MHz, Set to 3 for160 MHz or 80 + 80 MHz. Grouping 2 Indicates the subcarrier grouping,Ng, used for the compressed beamforming feedback matrix: Set to 0 for Ng= 1 (No grouping), Set to 1 for Ng = 2, Set to 2 for Ng = 4, The value 3is reserved. Codebook 1 Indicates the size of codebook entries:Information If Feedback Type is SU: Set to 0 for bψ = 2 and bΦ = 4, Setto 1 for bψ = 4 and bΦ = 6. If Feedback Type is MU: Set to 0 for bψ = 5and bΦ = 7 Set to 1 for bψ = 7 and bΦ = 9. Here, bψ and bΦ indicate thenumber of quantization bits. Feedback 1 Indicates the feedback type:Type Set to 0 for SU-MIMO, Set to 1 for MU-MIMO. Remaining 3 Indicatesthe number of remaining feedback Feedback segments for the associatedVHT Compressed Segments Beamforming frame: Set to 0 for the lastfeedback segment of a segmented report or the only feedback segment ofan unsegmented report. Set to a value between 1 and 6 for a feedbacksegment that is neither the first nor the last of a segmented report.Set to a value between 1 and 6 for a feedback segment that is not thelast feedback segment of a segmented report. In a retransmitted feedbacksegment, the field is set to the same value as the associated feedbacksegment in the original transmission. First 1 Set to 1 for the firstfeedback segment of a Feedback segmented report or the only feedbacksegment of Segment an unsegmented report; Set to 0 if it is not thefirst feedback segment or if the VHT Compressed Beamforming Report fieldand MU Exclusive Beamforming Report field are not present in the frame.In a retransmitted feedback segment, the field is set to the same valueas the associated feedback segment in the original transmission.Sounding 6 Set to the value of the sounding dialog token of the DialogNDPA frame. Token Number

In a VHT Compressed Beamforming frame not carrying all or part of theVHT Compressed Beamforming Report field, the Nc Index subfield, Nr Indexsubfield, Channel Width subfield, Grouping subfield, CodebookInformation subfield, Feedback Type subfield, and Sounding Dialog TokenNumber field are reserved, the First Feedback Segment field is set to 0,and the Remaining Feedback Segments field is set to 7.

The Sounding Dialog Token Number field also may be called a SoundingSequence Number subfield.

2) The VHT Compressed Beamforming Report field is used to carry explicitfeedback information in the form of angles representing compressedbeamforming feedback matrices V for use by a transmit beamformer todetermine steering matrices Q.

Table 7 shows the subfields of the VHT Compressed Beamforming Reportfield.

TABLE 7 Subfield Bits Description Average SNR of 8 Signal-to-noise ratioat the beamformee Space-Time Stream 1 for space-time stream 1 averagedover all subcarriers . . . . . . . . . Average SNR of 8 Signal-to-noiseratio at the beamformee Space-Time Stream Nc for space-time stream Ncaveraged over all subcarriers Compressed Na × (bψ + bΦ)/2 Order ofangles in the Compressed Beamforming Beamforming feedback matrix for theFeedback Matrix V for corresponding subcarrier subcarrier k = scidx(0)Compressed Na × (bψ + bΦ)/2 Order of angles in the CompressedBeamforming Beamforming feedback matrix for the Feedback Matrix V forcorresponding subcarrier subcarrier k = scidx(1) . . . . . . . . .Compressed Na × (bψ + bΦ)/2 Order of angles in the CompressedBeamforming Beamforming feedback matrix for the Feedback Matrix V forcorresponding subcarrier subcarrier k = scidx(Ns − 1)

With reference to Table 7, the VHT compressed beamforming report fieldmay include the average SNR of each space-time stream and a CompressedBeamforming Feedback Matrix V for each subcarrier. The CompressedBeamforming Feedback Matrix is a matrix including information aboutchannel state and can be used to calculate a channel matrix (i.e.,steering matrix Q) for an MIMO-based transmission method.

scidx( ) refers to subcarriers which transmit the Compressed BeamformingFeedback Matrix subfield. Na is fixed by the Nr×Nc value (e.g., φ11,Ψ21, . . . for Nr×Nc=2×1).

Ns refers to the number of subcarriers which transmit a compressedbeamforming feedback matrix to the beamformer. A beamformee, by using agrouping method, can reduce the number of subcarriers Ns which transmitthe compressed beamforming feedback matrix. For example, the number ofbeamforming feedback matrices provided as feedback information can bereduced by grouping a plurality of subcarriers into one group andtransmitting a compressed beamforming feedback matrix for thecorresponding group. Ns may be calculated from the Channel Width andGrouping subfields in the VHT MIMO Control field.

Table 8 illustrates the average SNR of Space-Time Stream subfield.

TABLE 8 Average SNR of Space-Time i subfield AvgSNR_i −128 ≦10 dB −127−9.75 dB  −126 −9.5 dB . . . . . . +126 53.5 dB +127 ≧53.75 dB  

With reference to Table 8, an average SNR for each stream-space streamis obtained by calculating the average SNR of all subcarriers in thecorresponding channel and mapping the calculated average SNR into therange of −128 to +128.

3) The MU Exclusive Beamforming Report field is used to carry explicitfeedback information in the form of delta ( ) SNRs. The information inthe VHT Compressed Beamforming Report field and the MU ExclusiveBeamforming Report field can be used by an MU beamformer to determinesteering matrices Q.

Table 9 shows the subfields of the MU Exclusive Beamforming Report fieldincluded in a VHT compressed beamforming frame.

TABLE 9 Subfield Bits Description Delta SNR for 4 The deviation betweenthe SNR of the space-time stream 1 corresponding subcarrier and theaverage for subcarrier k = SNR of all subcarriers for the sscidx(0)corresponding space-time stream. . . . . . . Delta SNR for 4 Thedeviation between the SNR of the space-time stream Nc correspondingsubcarrier and the average for subcarrier k = SNR of all subcarriers forthe sscidx(0) corresponding space-time stream. . . . . . . Delta SNR for4 The deviation between the SNR of the space-time stream 1 correspondingsubcarrier and the average for subcarrier k = SNR of all subcarriers forthe sscidx(1) corresponding space-time stream. . . . . . . Delta SNR for4 The deviation between the SNR of the space-time stream Nccorresponding subcarrier and the average for subcarrier k = SNR of allsubcarriers for the sscidx(1) corresponding space-time stream. . . . . .. Delta SNR for 4 The deviation between the SNR of the space-time stream1 corresponding subcarrier and the average for subcarrier k = SNR of allsubcarriers for the sscidx(Ns′ − 1) corresponding space-time stream. . .. . . . Delta SNR for 4 The deviation between the SNR of the space-timestream Nc corresponding subcarrier and the average for subcarrier k =SNR of all subcarriers for the sscidx(Ns′ − 1) corresponding space-timestream.

With reference to Table 9, the MU Exclusive Beamforming Report field mayinclude an SNR for each space-time stream for each subcarrier.

Each Delta SNR subfield has a value which is in the range −8 dB to 7 dBin 1 dB increments.

scidx( ) refers to subcarrier(s) which transmit the Delta SNR subfield.

Ns refers to the number of subcarriers which transmit the Delta SNRsubfield to the beamformer.

FIG. 15 is a diagram illustrating a Beamforming Report Poll frame formatin a wireless communication system to which the present invention may beapplied.

Referring to FIG. 15, the Beamforming Report Poll frame consists of aFrame Control field, a Duration field, an RA (Receiving Address) field,a TA (Transmitting Address) field, a Feedback Segment RetransmissionBitmap field, and an FCS.

The RA field value is the address of the intended recipient.

The TA field value is the address of the STA transmitting the

Beamforming Report Poll or a bandwidth signaling TA.

The Feedback Segment Retransmission Bitmap field indicates the requestedfeedback segments of a VHT Compressed Beamforming report.

If the bit in position n (n=0 for LSB and n=7 for MSB) is 1, then thefeedback segment with the Remaining Feedback Segments subfield in theVHT MIMO Control field equal to n is requested. If the bit in position nis 0, then the feedback segment with the Remaining Feedback Segmentssubfield in the VHT MIMO Control field equal to n is not requested.

Group ID

Since a VHT WLAN system supports MU-MIMO transmission for higherthroughput, an AP may transmit a data frame simultaneously to at leastone MIMO-paired STA. The AP may transmit data simultaneously to an STAgroup including at least one STA associated with it. For example, themaximum number of paired STAs may be 4. When the maximum number ofspatial streams is 8, up to 4 spatial streams may be allotted to eachSTA.

In a WLAN system supporting Tunneled Direct Link Setup (TDLS), DirectLink Setup (DLS), or a mesh network, an STA trying to send data may senda PPDU to a plurality of STAs by using the MU-MIMO transmission scheme.

An example in which an AP sends a PPDU to a plurality of STAs accordingto the MU-MIMO transmission scheme is described below.

An AP transmits a PPDU simultaneously to paired STAs belonging to atransmission target STA group through different spatial streams. Asdescribed above, the VHT-SIG-A field of the VHT PPDU format includesGroup ID information and space-time stream information. Thus, each STAmay determine whether a PPDU is sent to itself. No spatial streams maybe assigned to particular STAs in the transmission target STA group andtherefore no data will be transmitted.

A Group ID Management frame is used to assign or change a user positioncorresponding to one or more group IDs. That is, the AP may inform ofSTAs connected to a particular group ID through the Group ID Managementframe before performing a MU-MIMO transmission.

FIG. 16 is a diagram illustrating a Group ID Management frame in awireless communication system to which the present invention may beapplied.

Referring to FIG. 16, the Group ID Management frame is a VHT Actionframe for supporting VHT functionality, and its frame body includes anAction field. The Action field is included in the frame body of a MACframe to provide a mechanism for specifying extended management actions.

The Action field consists of a Category field, a VHT Action field, a VHTMIMO Control field, a Membership Status Array field, and a User PositionArray field.

The Category field is set to a value indicating the VHT category (i.e.,VHT Action frame), and the VHT Action field is set to a value indicatingthe Group ID Management frame.

The Membership Status Array field consists of a 1-bit Membership Statussubfield for each group. If the Membership Status subfield is set to 0,this indicates that the STA is not a member of the group, and if theMembership Status subfield is set to 1, this indicates that the STA is amember of the group. By setting one or more Membership Status subfieldsin the Membership Status Array field to 1, one or more groups may beassigned to the STA.

The STA may have a user position in each group to which it belongs.

The User Position Array field consists of a 2-bit User Position subfieldfor each group. The user position of an STA in a group to which itbelongs is indicated by the User Position subfield in the User PositionArray field. An AP may assign the same user position to different STAsin each group.

An AP may transmit a Group ID Management frame only if thedot11VHTOptionImplemented parameter is true. The Group ID Managementframe shall be sent only to VHT STAs that have the MU Beamformee Capablefield in the VHT Capabilities element field set to 1. The Group IDManagement frame shall be sent as an individually addressed frame.

An STA receives a Group ID Management frame with an RA field matchingits MAC address. The STA updates GROUP_ID_MANAGEMENT, a PHYCONFIG_VECTORparameter, based on the content of the received Group ID Managementframe.

Transmission of a Group ID Management frame to a STA and any associatedacknowledgement from the STA shall be complete before the transmissionof an MU PPDU to the STA.

An MU PPDU shall be transmitted to a STA based on the content of theGroup ID Management frame that is most recently transmitted to the STAand for which an ACK is received.

Downlink (DL) MU-MIMO Frame

FIG. 17 is a diagram illustrating a DL multi-user (MU) PPDU format in awireless communication system to which an embodiment of the presentinvention may be applied.

Referring to FIG. 17, the PPDU is configured to include a preamble and adata field. The data field may include a service field, a scrambled PSDUfield, tail bits, and padding bits.

An AP may aggregate MPDUs and transmit a data frame using an aggregatedMPDU (A-MPDU) format. In this case, a scrambled PSDU field may includethe A-MPDU.

The A-MPDU includes a sequence of one or more A-MPDU subframes.

In the case of a VHT PPDU, the length of each A-MPDU subframe is amultiple of 4 octets. Accordingly, an A-MPDU may include an end-of-frame(EOF) pad of 0 to 3 octets after the last A-MPDU subframe in order tomatch the A-MPDU up with the last octet of a PSDU.

The A-MPDU subframe includes an MPDU delimiter, and an MPDU may beoptionally included after the MPDU delimiter. Furthermore, a pad octetis attached to the MPDU in order to make the length of each A-MPDUsubframe in a multiple of 4 octets other than the last A-MPDU subframewithin one A-MPDU.

The MPDU delimiter includes a reserved field, an MPDU length field, acyclic redundancy check (CRC) field, and a delimiter signature field.

In the case of a VHT PPDU, the MPDU delimiter may further include anend-of-frame (EOF) field. If an MPDU length field is 0 and an A-MPDUsubframe or A-MPDU used for padding includes only one MPDU, in the caseof an A-MPDU subframe on which a corresponding MPDU is carried, the EOFfield is set to “1.” If not, the EOF field is set to “0.”

The MPDU length field includes information about the length of the MPDU.

If an MPDU is not present in a corresponding A-MPDU subframe, the PDUlength field is set to “0.” An A-MPDU subframe in which an MPDU lengthfield has a value of “0” is used to be padded to a corresponding A-MPDUin order to match the A-MPDU up with available octets within a VHT PPDU.

The CRC field includes CRC information for an error check. The delimitersignature field includes pattern information used to search for an MPDUdelimiter.

Furthermore, the MPDU includes an MAC header, a frame body, and a framecheck sequence (FCS).

FIG. 18 is a diagram illustrating a DL multi-user (MU) PPDU format in awireless communication system to which an embodiment of the presentinvention may be applied.

In FIG. 18, the number of STAs receiving a corresponding PPDU is assumedto be 3 and the number of spatial streams allocated to each STA isassumed to be 1, but the number of STAs paired with an AP and the numberof spatial streams allocated to each STA are not limited thereto.

Referring to FIG. 18, the MU PPDU is configured to include L-TFs (i.e.,an L-STF and an L-LTF), an L-SIG field, a VHT-SIG-A field, a VHT-TFs(i.e., a VHT-STF and a VHT-LTF), a VHT-SIG-B field, a service field, oneor more PSDUs, a padding field, and a tail bit. The L-TFs, the L-SIGfield, the VHT-SIG-A field, the VHT-TFs, and the VHT-SIG-B field are thesame as those of FIG. 4, and a detailed description thereof is omitted.

Information for indicating PPDU duration may be included in the L-SIGfield. In the PPDU, PPDU duration indicated by the L-SIG field includesa symbol to which the VHT-SIG-A field has been allocated, a symbol towhich the VHT-TFs have been allocated, a field to which the VHT-SIG-Bfield has been allocated, bits forming the service field, bits forming aPSDU, bits forming the padding field, and bits forming the tail field.An STA receiving the PPDU may obtain information about the duration ofthe PPDU through information indicating the duration of the PPDUincluded in the L-SIG field.

As described above, group ID information and time and spatial streamnumber information for each user are transmitted through the VHT-SIG-A,and a coding method and MCS information are transmitted through theVHT-SIG-B. Accordingly, beamformees may check the VHT-SIG-A and theVHT-SIG-B and may be aware whether a frame is an MU MIMO frame to whichthe beamformee belongs. Accordingly, an STA which is not a member STA ofa corresponding group ID or which is a member of a corresponding groupID, but in which the number of streams allocated to the STA is “0” isconfigured to stop the reception of the physical layer to the end of thePPDU from the VHT-SIG-A field, thereby being capable of reducing powerconsumption.

In the group ID, an STA can be aware that a beamformee belongs to whichMU group and it is a user who belongs to the users of a group to whichthe STA belongs and who is placed at what place, that is, that a PPDU isreceived through which stream by previously receiving a group IDmanagement frame transmitted by a beamformer.

All of MPDUs transmitted within the VHT MU PPDU based on 802.11ac areincluded in the A-MPDU. In the data field of FIG. 18, each VHT A-MPDUmay be transmitted in a different stream.

In FIG. 18, the A-MPDUs may have different bit sizes because the size ofdata transmitted to each STA may be different.

In this case, null padding may be performed so that the time when thetransmission of a plurality of data frames transmitted by a beamformeris ended is the same as the time when the transmission of a maximuminterval transmission data frame is ended. The maximum intervaltransmission data frame may be a frame in which valid downlink data istransmitted by a beamformer for the longest time. The valid downlinkdata may be downlink data that has not been null padded. For example,the valid downlink data may be included in the A-MPDU and transmitted.Null padding may be performed on the remaining data frames other thanthe maximum interval transmission data frame of the plurality of dataframes.

For the null padding, a beamformer may fill one or more A-MPDUsubframes, temporally placed in the latter part of a plurality of A-MPDUsubframes within an A-MPDU frame, with only an MPDU delimiter fieldthrough encoding. An A-MPDU subframe having an MPDU length of 0 may becalled a null subframe.

As described above, in the null subframe, the EOF field of the MPDUdelimiter is set to “1.” Accordingly, when the EOF field set to 1 isdetected in the MAC layer of an STA on the receiving side, the receptionof the physical layer is stopped, thereby being capable of reducingpower consumption.

Block Ack Procedure

FIG. 19 is a diagram illustrating a downlink MU-MIMO transmissionprocess in a wireless communication system to which the presentinvention may be applied.

MI-MIMO in 802.11ac works only in the downlink direction from the AP toclients. A multi-user frame can be transmitted to multiple receivers atthe same time, but the acknowledgements must be transmitted individuallyin the uplink direction.

Every MPDU transmitted in a VHT MU PPDU based on 802.11ac is included inan A-MPDU, so responses to A-MPDUs within the VHT MU PPDU that are notimmediate responses to the VHT MU PPDU are transmitted in response toBAR (Block Ack Request) frames by the AP.

To begin with, the AP transmits a VHT MU PPDU (i.e., a preamble anddata) to every receiver (i.e., STA 1, STA 2, and STA 3). The VHT MU PPDUincludes VHT A-MPDUs that are to be transmitted to each STA.

Having received the VHT MU PPDU from the AP, STA 1 transmits a BA (BlockAcknowledgement) frame to the AP after an SIFS. A more detaileddescription of the BA frame will be described later.

Having received the BA from STA 1, the AP transmits a BAR (blockacknowledgement request) frame to STA2 after an SIFS, and STA2 transmitsa BA frame to the AP after an SIFS. Having received the BA frame fromSTA 2, the AP transmits a BAR frame to STA 3 after an SIFS, and STA 3transmits a BA frame to the AP after an SIFS.

When this process is performed all STAs, the AP transmits the next MUPPDU to all the STAs.

ACK (Acknowledqement)/Block ACK Frames

In general, an ACK frame is used as a response to an MPDU, and a blockACK frame is used as a response to an A-MPDU.

FIG. 20 is a diagram illustrating an ACK frame in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 20, the ACK frame consists of a Frame Control field, aDuration field, an RA field, and a FCS.

The RA field is set to the value of the Address 2 field of theimmediately preceding Data frame, Management frame, Block Ack Requestframe, Block Ack frame, or PS-Poll frame.

For ACK frames sent by non-QoS STAs, if the More Fragments subfield isset to 0 in the Frame Control field of the immediately preceding Data orManagement frame, the duration value is set to 0.

For ACK frames not sent by non-QoS STAs, the duration value is set tothe value obtained from the Duration/ID field of the immediatelypreceding Data, Management, PS-Poll, BlockAckReq, or BlockAck frameminus the time, in microseconds, required to transmit the ACK frame andits SIFS interval. If the calculated duration includes a fractionalmicrosecond, that value is rounded up to the next higher integer.

Hereinafter, the Block Ack Request frame will be discussed.

FIG. 21 is a diagram illustrating a Block Ack Request frame in awireless communication system to which the present invention may beapplied.

Referring to FIG. 21, the Block Ack Request frame consists of a FrameControl field, a Duration/ID field, an RA field, a TA field, a BARControl field, a BAR Information field, and a frame check sequence(FCS).

The RA field may be set to the address of the STA receiving the BARframe.

The TA field may be set to the address of the STA transmitting the BARframe.

The BAR Control field includes a BAR Ack Policy subfield, a Multi-TIDsubfield, a Compressed Bitmap subfield, a Reserved subfield, and a TIDInfo subfield.

Table 10 shows the BAR Control field.

TABLE 10 Subfield Bits Description BAR Ack 1 Set to 0 when the senderrequires immediate ACK Policy of a data transmission. Set to 1 when thesender does not require immediate ACK of a data transmission. Multi-TID1 Indicates the type of the BAR frame depending on Compressed 1 thevalues of the Multi-TID subfield and Bitmap Compressed Bitmap subfield.00: Basic BAR 01: Compressed BAR 10: Reserved 11: Multi-TID BAR Reserved9 TID_Info 4 The meaning of the TID_Info field depends on the type ofthe BAR frame. For a Basic BAR frame and a Compressed BAR frame, thissubfield contains information on TIDs for which a BA frame is required.For a Multi-TID BAR frame, this subfield contains the number of TIDs.

The BAR Information field contains different information depending onthe type of the BAR frame. This will be described with reference to FIG.22.

FIG. 22 is a diagram illustrating the BAR Information field of a BlockAck Request frame in a wireless communication system to which thepresent invention may be applied.

(a) of FIG. 22 illustrates the BAR Information field of Basic BAR andCompressed BAR frames, and (b) of FIG. 22 illustrates the BARInformation field of a Multi-TID BAR frame.

Referring to (a) of FIG. 22, for the Basic BAR and Compressed BARframes, the BAR Information field includes a Block Ack Starting SequenceControl subfield.

The Block Ack Starting Sequence Control subfield includes a FragmentNumber subfield and a Starting Sequence Number subfield.

The Fragment Number subfield is set to 0.

For the Basic BAR frame, the Starting Sequence Number subfield containsthe sequence number of the first MSDU for which the corresponding BARframe is sent. For the Compressed BAR frame, the Starting SequenceControl subfield contains the sequence number of the first MSDU orA-MSDU for which the corresponding BAR frame is sent.

Referring to (b) of FIG. 22, for the Multi-TID BAR frame, the BARInformation field includes a Per TID Info subfield and a Block AckStarting Sequence Control subfield, which are repeated for each TID.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield contains a TID value.

As described above, the Block Ack Starting Sequence Control subfieldincludes fragment Number and Starting Sequence Number subfields. TheFragment Number subfield is set to 0. The Starting Sequence Controlsubfield contains the sequence number of the first MSDU or A-MSDU forwhich the corresponding BAR frame is sent.

FIG. 23 is a diagram illustrating a Block Ack frame in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 23, the Block Ack (BA) frame consists of a FrameControl field, a Duration/ID field, an RA field, a TA field, a BAControl field, a BA Information field, and a Frame Check Sequence (FCS).

The RA field may be set to the address of the STA requesting the BAframe.

The TA field may be set to the address of the STA transmitting the BAframe.

The BA Control field includes a BA Ack Policy subfield, a Multi-TIDsubfield, a Compressed Bitmap subfield, a Reserved subfield, and aTID_Info subfield.

Table 11 shows the BA Control field.

TABLE 11 Subfield Bits Description BA Ack 1 Set to 0 when the senderrequires immediate ACK Policy of a data transmission. Set to 1 when thesender does not require immediate ACK of a data transmission. Multi-TID1 Indicates the type of the BA frame depending on the Compressed 1values of the Multi-TID subfield and Bitmap Compressed Bitmap subfield.00: Basic BA 01: Compressed BA 10: Reserved 11: Multi-TID BA Reserved 9TID_Info 4 The meaning of the TID_Info field depends on the type of theBA frame. For a Basic BA frame and a Compressed BA frame, this subfieldcontains information on TIDs for which a BA frame is required. For aMulti-TID BA frame, this subfield contains the number of TIDs.

The BA Information field contains different information depending on thetype of the BA frame. This will be described with reference to FIG. 24.

FIG. 24 is a diagram illustrating the BA Information field of a BlockAck frame in a wireless communication system to which the presentinvention may be applied.

(a) of FIG. 24 illustrates the BA Information field of a Basic BA frame,(b) of FIG. 24 illustrates the BA Information field of a Compressed BARframe, and (c) of FIG. 24 illustrates the BA Information field of aMulti-TID BA frame.

Referring to (a) of FIG. 24, for the Basic BA frame, the BA Informationfield includes a Block Ack Starting Sequence Control subfield and aBlock Ack Bitmap subfield.

As described above, the Block Ack Starting Sequence Control subfieldincludes a Fragment Number subfield and a Starting Sequence Numbersubfield.

The Fragment Number subfield is set to 0.

The Starting Sequence Number subfield contains the sequence number ofthe first MSDU for which the corresponding BA frame is sent, and is setto the same value as the immediately preceding Basic BAR frame.

The Block Ack Bitmap subfield is 128 octets in length and is used toindicate the received status of a maximum of 64 MSDUs. If a bit of theBlock Ack Bitmap subfield has a value of ‘1’, it indicates thesuccessful reception of a single MSDU corresponding to that bitposition, and if a bit of the Block Ack Bitmap subfield has a value of‘0’, it indicates the unsuccessful reception of a single MSDUcorresponding to that bit position.

Referring to (b) of FIG. 24, for the Compressed BA frame, the BAInformation field includes a Block Ack Starting Sequence Controlsubfield and a Block Ack Bitmap subfield.

As described above, the Block Ack Starting Sequence Control subfieldincludes a Fragment Number subfield and a Starting Sequence Numbersubfield.

The Fragment Number subfield is set to 0.

The Starting Sequence Number subfield contains the sequence number ofthe first MSDU or A-MSDU for which the corresponding BA frame is sent,and is set to the same value as the immediately preceding Basic BARframe.

The Block Ack Bitmap subfield is 8 octets in length and is used toindicate the received status of a maximum of 64 MSDUs and A-MSDU. If abit of the Block Ack Bitmap subfield has a value of ‘1’, it indicatesthe successful reception of a single MSDU or A-MSDU corresponding tothat bit position, and if a bit of the Block Ack Bitmap subfield has avalue of ‘0’, it indicates the unsuccessful reception of a single MSDUor A-MSDU corresponding to that bit position.

Referring to (c) of FIG. 24, for the Multi-TID BA frame, the BAInformation field includes a Per TID Info subfield and a Block AckStarting Sequence Control subfield, which are repeated for each TID inorder of increasing TID.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield contains a TID value.

As described above, the Block Ack Starting Sequence Control subfieldincludes fragment Number and Starting Sequence Number subfields. TheFragment Number subfield is set to 0. The Starting Sequence Controlsubfield contains the sequence number of the first MSDU or A-MSDU forwhich the corresponding BA frame is sent.

The Block Ack Bitmap subfield is 8 octets in length. If a bit of theBlock Ack Bitmap subfield has a value of ‘1’, it indicates thesuccessful reception of a single MSDU or A-MSDU corresponding to thatbit position, and if a bit of the Block Ack Bitmap subfield has a valueof ‘0’, it indicates the unsuccessful reception of a single MSDU orA-MSDU corresponding to that bit position.

UL Multiple User (MU) Transmission Method

A new frame format and numerology for an 802.11ax system, that is, thenext-generation WLAN system, are actively discussed in the situation inwhich vendors of various fields have lots of interests in thenext-generation Wi-Fi and a demand for high throughput and quality ofexperience (QoE) performance improvement are increased after 802.11ac.

IEEE 802.11ax is one of WLAN systems recently and newly proposed as thenext-generation WLAN systems for supporting a higher data rate andprocessing a higher user load, and is also called a so-called highefficiency WLAN (HEW).

An IEEE 802.11ax WLAN system may operate in a 2.4 GHz frequency band anda 5 GHz frequency band like the existing WLAN systems. Furthermore, theIEEE 802.11ax WLAN system may also operate in a higher 60 GHz frequencyband.

In the IEEE 802.11ax system, an FFT size four times larger than that ofthe existing IEEE 802.11 OFDM systems (e.g., IEEE 802.11a, 802.11n, and802.11ac) may be used in each bandwidth for average throughputenhancement and outdoor robust transmission for inter-symbolinterference. This is described below with reference to relateddrawings.

Hereinafter, in a description of an HE format PPDU according to anembodiment of the present invention, the descriptions of theaforementioned non-HT format PPDU, HT mixed format PPDU, HT-green fieldformat PPDU and/or VHT format PPDU may be reflected into the descriptionof the HE format PPDU although they are not described otherwise.

FIG. 25 is a diagram illustrating a high efficiency (HE) format PPDUaccording to an embodiment of the present invention.

FIG. 25(a) illustrates a schematic configuration of the HE format PPDU,and FIGS. 25(b) to 25(d) illustrate more detailed configurations of theHE format PPDU.

Referring to FIG. 25(a), the HE format PPDU for an HEW may basicallyinclude a legacy part (L-part: legacy-part), an HE-part, and an HE-datafield.

The L-part includes an L-STF, an L-LTF, and an L-SIG field as in a formmaintained in the existing WLAN system. The L-STF, the L-LTF, and theL-SIG field may be called a legacy preamble.

The HE-part is a part newly defined for the 802.11ax standard and mayinclude an HE-STF, a HE-SIG field, and an HE-LTF. In FIG. 25(a), thesequence of the HE-STF, the HE-SIG field, and the HE-LTF is illustrated,but the HE-STF, the HE-SIG field, and the HE-LTF may be configured in adifferent sequence. Furthermore, the HE-LTF may be omitted. Not only theHE-STF and the HE-LTF, but the HE-SIG field may be commonly called anHE-preamble.

Also, the L-part, HE-part (or HE-preamble) may be generally called aphysical (PHY) preamble.

The HE-SIG may include information (e.g., OFDMA, UL MU MIMO, andimproved MCS) for decoding the HE-data field.

The L-part and the HE-part may have different fast Fourier transform(FFT) sizes (i.e., different subcarrier spacing) and use differentcyclic prefixes (CPs).

In an 802.11ax system, an FFT size four times (4×) larger than that of alegacy WLAN system may be used. That is, the L-part may have a 1× symbolstructure, and the HE-part (more specifically, HE-preamble and HE-data)may have a 4× symbol structure. In this case, the FFT of a 1×, 2×, or 4×size means a relative size for a legacy WLAN system (e.g., IEEE 802.11a,802.11n, and 802.11ac).

For example, if the sizes of FFTs used in the L-part are 64, 128, 256,and 512 in 20 MHz, 40 MHz, 80 MHz, and 160 MHz, respectively, the sizesof FFTs used in the HE-part may be 256, 512, 1024, and 2048 in 20 MHz,40 MHz, 80 MHz, and 160 MHz, respectively.

If an FFT size is larger than that of a legacy WLAN system as describedabove, subcarrier frequency spacing is reduced. Accordingly, the numberof subcarriers per unit frequency is increased, but the length of anOFDM symbol is increased.

That is, if a larger FFT size is used, it means that subcarrier spacingis narrowed. Likewise, it means that an inverse discrete Fouriertransform (IDFT)/discrete Fourier transform (DFT) period is increased.In this case, the IDFT/DFT period may mean a symbol length other than aguard interval (GI) in an OFDM symbol.

Accordingly, if an FFT size four times larger than that of the L-part isused in the HE-part (more specifically, the HE-preamble and the HE-datafield), the subcarrier spacing of the HE-part becomes 1/4 times thesubcarrier spacing of the L-part, and the IDFT/DFT period of the HE-partis four times the IDFT/DFT period of the L-part. For example, if thesubcarrier spacing of the L-part is 312.5 kHz (=20 MHz/64, 40 MHz/128,80 MHz/256 and/or 160 MHz/512), the subcarrier spacing of the HE-partmay be 78.125 kHz (=20 MHz/256, 40 MHz/512, 80 MHz/1024 and/or 160MHz/2048). Furthermore, if the IDFT/DFT period of the L-part is 3.2 μs(=1/312.5 kHz), the IDFT/DFT period of the HE-part may be 12.8 μs(=1/78.125 kHz).

In this case, since one of 0.8 μs, 1.6 μs, and 3.2 μs may be used as aGI, the OFDM symbol length (or symbol interval) of the HE-part includingthe GI may be 13.6 μs, 14.4 μs, or 16 μs is depending on the GI.

Referring to FIG. 25 (b), the HE-SIG field may be divided into aHE-SIG-A field and a HE-SIG-B field.

For example, the HE-part of the HE format PPDU may include a HE-SIG-Afield having a length of 12.8 μs, an HE-STF of 1 OFDM symbol, one ormore HE-LTFs, and a HE-SIG-B field of 1 OFDM symbol.

Furthermore, in the HE-part, an FFT size four times larger than that ofthe existing PPDU may be applied from the HE-STF other than the HE-SIG-Afield. That is, FFTs having 256, 512, 1024, and 2048 sizes may beapplied from the HE-STFs of the HE format PPDUs of 20 MHz, 40 MHz, 80MHz, and 160 MHz, respectively.

In this case, if the HE-SIG field is divided into the HE-SIG-A field andthe HE-SIG-B field as in FIG. 25(b), the positions of the HE-SIG-A fieldand the HE-SIG-B field may be different from those of FIG. 25(b). Forexample, the HE-SIG-B field may be transmitted after the HE-SIG-A field,and the HE-STF and the HE-LTF may be transmitted after the HE-SIG-Bfield. In this case, an FFT size four times larger than that of theexisting PPDU may be applied from the HE-STF.

Referring to FIG. 25(c), the HE-SIG field may not be divided into aHE-SIG-A field and a HE-SIG-B field.

For example, the HE-part of the HE format PPDU may include an HE-STF of1 OFDM symbol, a HE-SIG field of 1 OFDM symbol, and one or more HE-LTFs.

In the manner similar to that described above, an FFT size four timeslarger than that of the existing PPDU may be applied to the HE-part.That is, FFT sizes of 256, 512, 1024, and 2048 may be applied from theHE-STF of the HE format PPDU of 20 MHz, 40 MHz, 80 MHz, and 160 MHz,respectively.

Referring to FIG. 25(d), the HE-SIG field is not divided into a HE-SIG-Afield and a HE-SIG-B field, and the HE-LTF may be omitted.

For example, the HE-part of the HE format PPDU may include an HE-STF of1 OFDM symbol and a HE-SIG field of 1 OFDM symbol.

In the manner similar to that described above, an FFT size four timeslarger than that of the existing PPDU may be applied to the HE-part.That is, FFT sizes of 256, 512, 1024, and 2048 may be applied from theHE-STF of the HE format PPDU of 20 MHz, 40 MHz, 80 MHz, and 160 MHz,respectively.

The HE format PPDU for the WLAN system to which the present inventionmay be applied may be transmitted through at least one 20 MHz channel.For example, the HE format PPDU may be transmitted in the 40 MHz, 80 MHzor 160 MHz frequency band through total four 20 MHz channel. This willbe described in more detail with reference to the drawing below.

FIG. 26 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

FIG. 26 illustrates a PPDU format when 80 MHz is allocated to one STA(or OFDMA resource units are allocated to multiple STAs within 80 MHz)or when different streams of 80 MHz are allocated to multiple STAs,respectively.

Referring to FIG. 26, an L-STF, an L-LTF, and an L-SIG may betransmitted an OFDM symbol generated on the basis of 64 FFT points (or64 subcarriers) in each 20 MHz channel.

Also, the HE-SIG B field may be positioned after the HE-SIG Afield. Inthis case, an FFT size per unit frequency may be further increased afterthe HE-SFT (or HE-SIG B). For example, from the HE-STF (or HE-SIG-B),256 FFTs may be used in the 20 MHz channel, 512 FFTs may be used in the40 MHz channel, and 1024 FFTs may be used in the 80 MHz channel.

A HE-SIG-A field may include common control information commonlyreceived by STAs which receive a PPDU. The HE-SIG-A field may betransmitted in 1 to 3 OFDM symbols. The HE-SIG-Afield is duplicated foreach 20

MHz and contains the same information. Also, the HE-SIG-A fieldindicates full bandwidth information of the system.

Table 12 illustrates information contained in the HE-SIG-A field.

TABLE 12 Field Bits Description Bandwidth 2 Indicates a bandwidth inwhich a PPDU is transmitted. For example, 20 MHz, 40 MHz, 80 MHz or 160MHz Group ID 6 Indicates an STA or a group of STAs that will receive aPPDU Stream 12 Indicates the number or location of spatial streams foreach information STA or the number or location of spatial streams for agroup of STAs UL indication 1 Indicates whether a PPDU is destined to anAP (uplink) or STA (downlink) MU 1 Indicates whether a PPDU is anSU-MIMO PPDU or an indication MU-MIMO PPDU GI indication 1 Indicateswhether a short GI or a long GI is used Allocation 12 Indicates a bandor a channel (subchannel index or information subband index) allocatedto each STA in a bandwidth in which a PPDU is transmitted Transmission12 Indicates a transmission power for each channel or each power STA

Information contained in each of the fields illustrated in Table 12 maybe as defined in the IEEE 802.11 system. Also, the above-describedfields are examples of the fields that may be included in the PPDU butnot limited to them. That is, the above-described fields may besubstituted with other fields or further include additional fields, andnot all of the fields may be necessarily included. Another example ofinformation included in the HE-SIG A field will be described hereinafterin relation to FIG. 34.

The HE-STF field is used to improve AGC estimation in MIMO transmission.

The HE-SIG-B field may include user-specific information that isrequired for each STA to receive its own data (i.e., a Physical LayerService Data Unit (PSDU)). The HE-SIG-B field may be transmitted in oneor two OFDM symbols. For example, the HE-SIG-B field may includeinformation about the length of a corresponding PSDU and the Modulationand Coding Scheme (MCS) of the corresponding PSDU.

The L-STF field, the L-LTF field, the L-SIG field, and the HE-SIG-Afield may be duplicately transmitted every 20 MHz channel. For example,when a PPDU is transmitted through four 20 MHz channels, the L-STFfield, the L-LTF field, L-STG field, and the HE-SIG-A field may beduplicately transmitted every 20 MHz channel.

If the FFT size is increased, a legacy STA that supports conventionalIEEE 802.11a/g/n/ac may be unable to decode a corresponding PPDU. Forcoexistence between a legacy STA and a HE STA, the L-STF, L-LTF, andL-SIG fields are transmitted through 64 FFT in a 20 MHz channel so thatthey can be received by a legacy STA. For example, the L-SIG field mayoccupy a single OFDM symbol, a single OFDM symbol time may be 4 μs, anda GI may be 0.8 μs.

An FFT size per unit frequency may be further increased from the HE-STF(or from the HE-SIG-A). For example, 256 FFT may be used in a 20 MHzchannel, 512 FFT may be used in a 40 MHz channel, and 1024 FFT may beused in an 80 MHz channel. If the FFT size is increased, the number ofOFDM subcarriers per unit frequency is increased because spacing betweenOFDM subcarriers is reduced, but an OFDM symbol time may be increased.In order to improve system efficiency, the length of a GI after theHE-STF may be set equal to the length of the GI of the HE-SIG-A.

The HE-SIG-A field includes information that is required for a HE STA todecode a HE PPDU. However, the HE-SIG-A field may be transmitted through64 FFT in a 20 MHz channel so that it may be received by both a legacySTA and a HE STA. The reason for this is that a HE STA is capable ofreceiving conventional HTNHT format PPDUs in addition to a HE formatPPDU. In this case, it is required that a legacy STA and a HE STAdistinguish a HE format PPDU from an HTNHT format PPDU, and vice versa.

FIG. 27 is a drawing illustrating an HE format PPDU according to anembodiment of the present invention.

In FIG. 27, it is assumed that 20 MHz channels are allocated todifferent STAs (e.g., STA 1, STA 2, STA 3, and STA 4).

Referring to FIG. 27, an FFT size per unit frequency may be furtherincreased from the HE-SFT (or the HE-SIG-B). For example, from theHE-STF (or HE-SIG-B), 256 FFTs may be used in the 20 MHz channel, 512FFTs may be used in the 40 MHz channel, and 1024 FFTs may be used in the80 MHz channel.

Information transmitted in each field included in a PPDU is the same asthe example of FIG. 26, and thus, descriptions thereof will be omittedhereinafter.

The HE-SIG-B may include information specified to each STA but it may beencoded in the entire band (i.e., indicated in the HE-SIG-A field). Thatis, the HE-SIG-B field includes information regarding every STA andevery STA receives the HE-SIG-B field.

The HE-SIG-B field may provide frequency bandwidth information allocatedto each STA and/or stream information in a corresponding frequency band.For example, in FIG. 27, as for the HE-SIG-B, STA 1 may be allocated 20MHz, STA 2 may be allocated a next 20 MHz, STA 3 may be allocated a next20 MHz, and STA 4 may be allocated a next 20 MHz. Also, the STA 1 andSTA 2 may be allocated 40 MHz and STA 3 and STA 4 may be allocated anext 40 MHz. In this case, STA 1 and STA 2 may be allocated differentstreams and STA 3 and STA 4 may be allocated different streams.

Also, an HE-SIG C field may be defined and added to the example of FIG.27. Here, information regarding every STA may be transmitted in theentire band in the HE-SIG-B field, and control information specified toeach STA may be transmitted by 20 MHz through the HE-SIG-C field.

Also, unlike the examples of FIGS. 26 and 27, the HE-SIG-B field may notbe transmitted in the entire band but may be transmitted by 20 MHz, likethe HE-SIG-A field. This will be described with reference to FIG. 26.

FIG. 28 is a diagram illustrating an HE format PPDU according to anembodiment of the present invention.

In FIG. 28, it is assumed that 20 MHz channels are allocated todifferent STAs (e.g., STA 1, STA 2, STA 3, and STA 4).

Referring to FIG. 28, the HE-SIG-B field is not transmitted in theentire band but is transmitted by 20 MHz, like the HE-SIG-A field. Here,however, unlike the HE-SIG-A field, the HE-SIG-B field may be encoded by20 MHz and transmitted but may not be duplicated by 20 MHz andtransmitted.

Here, an FFT size per unit frequency may be further increased from theHE-STF (or the HE-SIG-B). For example, from the HE-STF (or HE-SIG-B),256 FFTs may be used in the 20 MHz channel, 512 FFTs may be used in the40 MHz channel, and 1024 FFTs may be used in the 80 MHz channel.

Information transmitted in each field included in the PPDU is the sameas the example of FIG. 26, and thus, descriptions thereof will beomitted.

The HE-SIG-A field is duplicated by 20 MHz and transmitted.

The HE-SIG-B field may provide frequency bandwidth information allocatedto each STA and/or stream information in a corresponding frequency band.Since the HE-SIG-B field includes information regarding each STA,information regarding each STA may be included in each HE-SIG-B field inunits of 20 MHz. Here, in the example of FIG. 28, 20 MHz is allocated toeach STA, but, in a case in which 40 MHz is allocated to an STA, theHE-SIG-B may be duplicated by 20 MHz and transmitted.

In a case where a partial bandwidth having a low level of interferencefrom an adjacent BSS is allocated to an STA in a situation in which eachBSS supports different bandwidths, the HE-SIG-B is preferably nottransmitted in the entire band as mentioned above.

Hereinafter, the HE format PPDU of FIG. 28 will be described for thepurposes of description.

In FIGS. 26 to 28, a data field, as payload, may include a servicefield, a scrambled PSDU, a tail bit, and a padding bit.

Meanwhile, the HE format PPDU illustrated in FIGS. 26 to 28 may bedistinguished through a repeated L-SIG (RL-SIG), a repeated symbol of anL-SIG field. The RL-SIG field is inserted in front of the HE SIG-Afield, and each STA may identify a format of a received PPDU using theRL-SIG field, as an HE format PPDU.

A multi-user UL transmission method in a WLAN system is described below.

A method of transmitting, by an AP operating in a WLAN system, data to aplurality of STAs on the same time resource may be called downlinkmulti-user (DL MU) transmission. In contrast, a method of transmitting,by a plurality of STAs operating in a WLAN system, data to an AP on thesame time resource may be called uplink multi-user (UL MU) transmission.

Such DL MU transmission or UL MU transmission may be multiplexed on afrequency domain or a space domain.

If DL MU transmission or UL MU transmission is multiplexed on thefrequency domain, different frequency resources (e.g., subcarriers ortones) may be allocated to each of a plurality of STAs as DL or ULresources based on orthogonal frequency division multiplexing (OFDMA). Atransmission method through different frequency resources in such thesame time resources may be called “DL/UL MU OFDMA transmission.”

If DL MU transmission or UL MU transmission is multiplexed on the spacedomain, different spatial streams may be allocated to each of aplurality of STAs as DL or UL resources. A transmission method throughdifferent spatial streams on such the same time resources may be called“DL/UL MU MIMO transmission.”

Current WLAN systems do not support UL MU transmission due to thefollowing constraints.

Current WLAN systems do not support synchronization for the transmissiontiming of UL data transmitted by a plurality of STAs. For example,assuming that a plurality of STAs transmits UL data through the sametime resources in the existing WLAN system, in the present WLAN systems,each of a plurality of STAs is unaware of the transmission timing of ULdata of another STA.

Accordingly, an AP may not receive UL data from each of a plurality ofSTAs on the same time resource.

Furthermore, in the present WLAN systems, overlap may occur betweenfrequency resources used by a plurality of STAs in order to transmit ULdata. For example, if a plurality of STAs has different oscillators,frequency offsets may be different. If a plurality of STAs havingdifferent frequency offsets performs UL transmission at the same timethrough different frequency resources, frequency regions used by aplurality of STAs may partially overlap.

Furthermore, in existing WLAN systems, power control is not performed oneach of a plurality of STAs. An AP dependent on the distance betweeneach of a plurality of STAs and the AP and a channel environment mayreceive signals of different power from a plurality of STAs. In thiscase, a signal having weak power may not be relatively detected by theAP compared to a signal having strong power.

Accordingly, an embodiment of the present invention proposes an UL MUtransmission method in a WLAN system.

FIG. 29 is a diagram illustrating an uplink multi-user transmissionprocedure according to an embodiment of the present invention.

Referring to FIG. 29, an AP may instruct STAs participating in UL MUtransmission to prepare for UL MU transmission, receive an UL MU dataframe from these STAs, and send an ACK frame (BA (Block Ack) frame) inresponse to the UL MU data frame.

First of all, the AP instructs STAs that will transmit UL MU data toprepare for UL MU transmission by sending an UL MU Trigger frame 2910.Here, the term UL MU scheduling frame may be called “UL MU schedulingframe”.

Here, the UL MU Trigger frame 2910 may contain control information suchas STA ID (identifier)/address information, information on theallocation of resources to be used by each STA, and durationinformation.

The STA ID/address information refers to information on the identifieror address for specifying an STA that transmits uplink data.

The resource allocation information refers to information on uplinktransmission resources allocated to each STA (e.g., information onfrequency/subcarriers allocated to each STA in the case of UL MU OFDMAtransmission and a stream index allocated to each STA in the case of ULMU MIMO transmission).

The duration information refers to information for determining timeresources for transmitting an uplink data frame sent by each of multipleSTAs.

For example, the duration information may include period information ofa TXOP (Transmit Opportunity) allocated for uplink transmission of eachSTA or information (e.g., bits or symbols) on the uplink frame length.

Also, the UL MU Trigger frame 2910 may further include controlinformation such as information on an MCS to be used when each STA sendsan UL MU data frame, coding information, etc.

The above-mentioned control information may be transmitted in a HE-part(e.g., the HE-SIG-A field or HE-SIG-B field) of a PPDU for deliveringthe UL MU Trigger frame 2910 or in the control field of the UL MUTrigger frame 2910 (e.g., the Frame Control field of the MAC frame).

The PPDU for delivering the UL MU Trigger frame 2910 starts with anL-part (e.g., the L-STF field, L-LTF field, and L-SIG field).Accordingly, legacy STAs may set their NAV (Network Allocation Vector)by L-SIG protection through the L-SIG field. For example, in the L-SIG,legacy STAs may calculate a period for NAV setting (hereinafter, ‘L-SIGprotection period’) based on the data length and data rate. The legacySTAs may determine that there is no data to be transmitted to themselvesduring the calculated L-SIG protection period.

For example, the L-SIG protection period may be determined as the sum ofthe value of the MAC Duration field of the UL MU Trigger frame 2910 andthe remaining portion after the L-SIG field of the PPDU delivering theUL MU Trigger frame 2910. Accordingly, the L-SIG protection period maybe set to a period of time until the transmission of an ACK frame 2930(or BA frame) transmitted to each STA, depending on the MAC durationvalue of the UL MU Trigger frame 2910.

Hereinafter, a method of resource allocation to each STA for UL MUtransmission will be described in more detail. Afield containing controlinformation will be described separately for convenience of explanation,but the present invention is not limited to this.

A first field may indicate UL MU OFDMA transmission and UL MU MIMOtransmission in different ways. For example, ‘0’ may indicate UL MUOFDMA transmission, and ‘1’ may indicate UL MU MIMO transmission. Thefirst field may be 1 bit in size.

A second field (e.g., STA ID/address field) indicates the IDs oraddresses of STAs that will participate in UL MU transmission. The sizeof the second field may be obtained by multiplying the number of bitsfor indicating an STA ID by the number of STAs participating in UL MU.For example, if the second field has 12 bits, the ID/address of each STAmay be indicated in 4 bits.

A third field (e.g., resource allocation field) indicates a resourceregion allocated to each STA for UL MU transmission. Each STA may besequentially informed of the resource region allocated to it accordingto the order in the second field.

If the first field has a value of 0, this indicates frequencyinformation (e.g., frequency index, subcarrier index, etc.) for UL MUtransmission in the order of STA IDs/addresses in the second field, andif the first field has a value of 1, this indicates MIMO information(e.g., stream index, etc.) for UL MU transmission in the order of STAIDs/addresses in the second field.

In this case, a single STA may be informed of multiple indices (i.e.,frequency/subcarrier indices or stream indices). Thus, the third fieldmay be configured by multiplying the number of bits (or which may beconfigured in a bitmap format) by the number of STAs participating in ULMU transmission.

For example, it is assumed that the second field is set in the order ofSTA 1, STA 2, . . . , and the third field is set in the order of 2, 2, .. . .

In this case, if the first field is 0, frequency resources may beallocated to STA 1 and STA2, sequentially in the order of higherfrequency region (or lower frequency region). In an example, when 20 MHzOFDMA is supported in an 80 MHz band, STA 1 may use a higher (or lower)40 MHz band and STA 2 may use the subsequent 40 MHz band.

On the other hand, if the first field is 1, streams may be allocated toSTA 1 and STA 2, sequentially in the order of higher-order (orlower-order) streams. In this case, a beamforming scheme for each streammay be prescribed, or the third field or fourth field may contain morespecific information on the beamforming scheme for each stream.

Each STA sends a UL MU Data frame 2921, 2922, and 2923 to an AP based onthe UL MU Trigger frame 2910. That is, each STA may send a UL MU Dataframe 2921, 2922, and 2923 to an AP after receiving the UL MU Triggerframe 2910 from the AP.

Each STA may determine particular frequency resources for UL MU OFDMAtransmission or spatial streams for UL MU MIMO transmission, based onthe resource allocation information in the UL MU Trigger frame 2910.

Specifically, for UL MU OFDMA transmission, each STA may send an uplinkdata frame on the same time resource through a different frequencyresource.

Here, each of STA 1 to STA 3 may be allocated different frequencyresources for uplink data frame transmission, based on the STAID/address information and resource allocation information included inthe UL MU Trigger frame 2910. For example, the STA ID/addressinformation may sequentially indicate STA 1 to STA 3, and the resourceallocation information may sequentially indicate frequency resource 1,frequency resource 2, and frequency resource 3. In this case, STA 1 toSTA 3 sequentially indicated based on the STA ID/address information maybe allocated frequency resource 1, frequency resource 2, and frequencyresource 3, which are sequentially indicated based on the resourceallocation information. That is, STA 1, STA 2, and STA 3 may send theuplink data frame 2921, 2922, and 2923 to the AP through frequencyresource 1, frequency resource 2, and frequency resource 3,respectively.

For UL MU MIMO transmission, each STA may send an uplink data frame onthe same time resource through at least one different stream among aplurality of spatial streams.

Here, each of STA 1 to STA 3 may be allocated spatial streams for uplinkdata frame transmission, based on the STA ID/address information andresource allocation information included in the UL MU Trigger frame2910. For example, the STA ID/address information may sequentiallyindicate STA 1 to STA 3, and the resource allocation information maysequentially indicate spatial stream 1, spatial stream 2, and spatialstream 3. In this case, STA 1 to STA 3 sequentially indicated based onthe STA ID/address information may be allocated spatial stream 1,spatial stream 2, and spatial stream 3, which are sequentially indicatedbased on the resource allocation information. That is, STA 1, STA 2, andSTA 3 may send the uplink data frame 2921, 2922, and 2923 to the APthrough spatial stream 1, spatial stream 2, and spatial stream 3,respectively.

The PPDU for delivering the uplink data frame 2921, 2922, and 2923 mayhave a new structure, even without an L-part.

For UL MU MIMO transmission or for UL MU OFDMA transmission in a subbandbelow 20 MHz, the L-part of the PPDU for delivering the uplink dataframe 2921, 2922, and 2923 may be transmitted on an SFN (that is, allSTAs send an L-part having the same configuration and content). On thecontrary, for UL MU OFDMA transmission in a subband above 20 MHz, theL-part of the PPDU for delivering the uplink data frame 2921, 2922, and2923 may be transmitted every 20 MHz.

As long as the information in the UL MU Trigger frame 2910 suffices toconstruct an uplink data frame, the HE-SIG field (i.e., a part wherecontrol information for a data frame configuration scheme istransmitted) in the PPDU delivering the uplink data frame 2921, 2922,and 2923 may not be required. For example, the HE-SIG-A field and/or theHE-SIG-B field may not be transmitted. Also, the HE-SIG-A field and theHE-SIG C field may be transmitted, but the HE-SIG-B field may not betransmitted.

An AP may send an ACK Frame 2930 (or BA frame) in response to the uplinkdata frame 2921, 2922, and 2923 received from each STA. Here, the AP mayreceive the uplink data frame 2921, 2922, and 2923 from each STA andthen, after an SIFS, transmit the ACK frame 2930 to each STA.

Using the existing ACK frame structure, an RA field having a size of 6octets may include the AID (or Partial AID) of STAs participating in ULMU transmission.

Alternatively, an ACK frame with a new structure may be configured forDL SU transmission or DL MU transmission.

The AP may send an ACK frame 2930 to an STA only when an UL MU dataframe is successfully received by the corresponding STA. Through the ACKframe 2930, the AP may inform whether the reception is successful or notby ACK or NACK. If the ACK frame 2930 contains NACK information, it alsomay include the reason for NACK or information (e.g., UL MU schedulinginformation, etc.) for the subsequent procedure.

Alternatively, the PPDU for delivering the ACK frame 2930 may beconfigured to have a new structure without an L-part.

The ACK frame 2930 may contain STA ID or address information, but theSTA ID or address information may be omitted if the order of STAsindicated in the UL MU Trigger frame 2910 also applies to the ACK frame2930.

Moreover, the TXOP (i.e., L-SIG protection period) of the ACK frame 2930may be extended, and a frame for the next UL MU scheduling or a controlframe containing adjustment information for the next UL MU transmissionmay be included in the TXOP.

Meanwhile, an adjustment process may be added to synchronize STAs for ULMU transmission.

FIGS. 30 to 32 are drawings illustrating a resource allocation unit inan OFDMA multi-user transmission scheme according to an embodiment ofthe present invention.

When a DL/UL OFDMA transmission scheme is used, a plurality of resourceunits may be defined in units of n tones (or subcarriers) within a PPDUbandwidth.

A resource unit refers to an allocation unit of frequency resource forDL/UL OFDMA transmission.

One or more resource units may be allocated as DL/UL frequency resourceto one STA and different resource units may be allocated to a pluralityof STAs.

FIG. 30 illustrates a case in which a PPDU bandwidth is 20 MHz.

Seven DC tones may be positioned in a central frequency region of the 20MHz PPDU bandwidth. Also, six left guard tones may and five right guardtones may be positioned on both sides of the 20 MHz PPDU bandwidth,respectively.

According to a resource unit configuration scheme such as that of FIG.30(a), one resource unit may be comprised of 26 tones. Also, accordingto a resource unit configuration scheme such as that of FIG. 30(b), oneresource unit may be comprised of 52 tone or 26 tones. Also, accordingto a resource unit configuration scheme such as that of FIG. 30(c), oneresource unit may be comprised of 106 tone or 26 tones. Also, accordingto a resource unit configuration scheme such as that of FIG. 30(d), oneresource unit may be comprised of 242 tones.

The resource unit comprised of 26 tones may include two pilot tones, theresource unit comprised of 52 tones may include four pilot tones, andthe resource unit comprised of 106 tones may include four pilot tones.

In a case where a resource unit is configured as illustrated in FIG.30(a), up to 9 STAs may be supported for DL/UL OFDMA transmission in the20 MHz band. Also, in a case where a resource unit is configured asillustrated in FIG. 30(b), up to 5 STAs may be supported for DL/UL OFDMAtransmission in the 20 MHz band. Also, in a case where a resource unitis configured as illustrated in FIG. 30(c), up to 3 STAs may besupported for DL/UL OFDMA transmission in the 20 MHz band. Also, in acase where a resource unit is configured as illustrated in FIG. 30(d),20 MHz band may be allocated to one STA.

On the basis of the number of STAs participating in DL/UL OFDMAtransmission and/or an amount of data transmitted or received by acorresponding STA, any one of the resource unit configuration schemesillustrated in FIGS. 30(a) to 30(d) may be applied or a combination ofthe resource unit configuration schemes of FIGS. 30(a) to 30(d) may beapplied.

FIG. 31 illustrates a case in which a PPDU bandwidth is 40 MHz.

Five DC tones may be positioned in a central frequency region of the 40MHz PPDU bandwidth. Also, 12 left guard tones and 11 right guard tonesmay be positioned on both sides of the 40 MHz PPDU bandwidth,respectively.

According to a resource unit configuration scheme illustrated in FIG.31(a), one resource unit may be comprised of 26 tones. Also, accordingto a resource unit configuration scheme illustrated in FIG. 31(b), oneresource unit may be comprised of 52 tones or 26 tones. Also, accordingto a resource unit configuration scheme illustrated in FIG. 31(c), oneresource unit may be comprised of 106 tones or 26 tones. Also, accordingto a resource unit configuration scheme illustrated in FIG. 31(d), oneresource unit may be comprised of 242 tones. Also, according to aresource unit configuration scheme illustrated in FIG. 31(e), oneresource unit may be comprised of 484 tones.

The resource unit comprised of 26 tones may include two pilot tones, theresource unit comprised of 52 tones may include four pilot tones, theresource unit comprised of 52 tones may include four pilot tones, theresource unit comprised of 106 tones may include four pilot tones, theresource unit comprised of 242 tones may include eight pilot tones, andthe resource unit comprised of 484 tones may include 16 pilot tones.

When a resource unit is configured as illustrated in FIG. 31(a), up to18 STAs may be supported for DL/UL OFDMA transmission in the 40 MHzband. Also, when a resource unit is configured as illustrated in FIG.31(b), up to 10 STAs may be supported for DL/UL OFDMA transmission inthe 40 MHz band. Also, when a resource unit is configured as illustratedin FIG. 31(c), up to 6 STAs may be supported for DL/UL OFDMAtransmission in the 40 MHz band. Also, when a resource unit isconfigured as illustrated in FIG. 31(d), up to 2 STAs may be supportedfor DL/UL OFDMA transmission in the 40 MHz band. Also, when a resourceunit is configured as illustrated in FIG. 31(e), a correspondingresource unit may be allocated to one STA for SU DL/UL transmission inthe 40 MHz band.

On the basis of the number of STAs participating in DL/UL OFDMAtransmission and/or an amount of data transmitted or received by acorresponding STA, any one of the resource unit configuration schemesillustrated in FIGS. 31(a) to 31(e) may be applied or a combination ofthe resource unit configuration schemes of FIGS. 31(a) to 31(e) may beapplied.

FIG. 32 illustrates a case in which a PPDU bandwidth is 80 MHz.

Seven DC tones may be positioned in a central frequency region of the 80MHz PPDU bandwidth. However, in a case where the 80 MHz PPDU bandwidthis allocated to one STA (that is, in a case where a resource unitcomprised of 996 tones is allocated to one STA), five DC tones may bepositioned in the central frequency region. Also, 12 left guard tonesand 11 right guard tones may be positioned on both sides of the 80 MHzPPDU bandwidth, respectively.

According to a resource unit configuration scheme illustrated in FIG.32(a), one resource unit may be comprised of 26 tones. Also, accordingto a resource unit configuration scheme illustrated in FIG. 32(b), oneresource unit may be comprised of 52 tones or 26 tones. Also, accordingto a resource unit configuration scheme illustrated in FIG. 32(c), oneresource unit may be comprised of 106 tones or 26 tones. Also, accordingto a resource unit configuration scheme illustrated in FIG. 32(d), oneresource unit may be comprised of 242 tones or 26. Also, according to aresource unit configuration scheme illustrated in FIG. 32(e), oneresource unit may be comprised of 484 tones or 26 tones. Also, accordingto a resource unit configuration scheme illustrated in FIG. 32(f), oneresource unit may be comprised of 996 tones.

The resource unit comprised of 26 tones may include two pilot tones, theresource unit comprised of 52 tones may include four pilot tones, theresource unit comprised of 52 tones may include four pilot tones, theresource unit comprised of 106 tones may include four pilot tones, theresource unit comprised of 242 tones may include eight pilot tones, theresource unit comprised of 484 tones may include 16 pilot tones, and theresource unit comprised of 996 tones may include 16 pilot tones.

When a resource unit is configured as illustrated in FIG. 32(a), up to37 STAs may be supported for DL/UL OFDMA transmission in the 80 MHzband. Also, when a resource unit is configured as illustrated in FIG.32(b), up to 21 STAs may be supported for DL/UL OFDMA transmission inthe 80 MHz band. Also, when a resource unit is configured as illustratedin FIG. 32(c), up to 13 STAs may be supported for DL/UL OFDMAtransmission in the 80 MHz band. Also, when a resource unit isconfigured as illustrated in FIG. 32(d), up to 5 STAs may be supportedfor DL/UL OFDMA transmission in the 80 MHz band. Also, when a resourceunit is configured as illustrated in FIG. 32(e), up to 3 STAs may besupported for DL/UL OFDMA transmission in the 80 MHz band. Also, when aresource unit is configured as illustrated in FIG. 32(f), acorresponding resource unit may be allocated to one STA for SU DL/ULtransmission in the 80 MHz band.

On the basis of the number of STAs participating in DL/UL OFDMAtransmission and/or an amount of data transmitted or received by acorresponding STA, any one of the resource unit configuration schemesillustrated in FIGS. 32(a) to 32(f) may be applied or a combination ofthe resource unit configuration schemes of FIGS. 32(a) to 32(f) may beapplied.

In addition, although not shown, a resource unit configuration scheme ina case where a PPDU bandwidth is 160 MHz may also be proposed. In thiscase, the 160 MHz PPDU bandwidth may have a structure in which theaforementioned 80 MHz PPDU bandwidth is repeated twice.

Among the entire resource units determined according to theaforementioned resource unit configuration schemes, only some resourceunits may be used for DL/UL OFDMA transmission. For example, in a casewhere resource units are configured as illustrated in FIG. 30(a) within20 MHz, one resource unit is allocated to each of less than 9 STAs andthe other resource units may not be allocated to any STA.

In the case of DL OFDMA transmission, a data field of a PPDU ismultiplexed in a frequency domain by the resource unit allocated to eachSTA and transmitted.

Meanwhile, in the case of UL OFDMA transmission, each STA may configurea data field of a PPDU by the resource unit allocated thereto andsimultaneously transmit the PPDU to an AP. In this manner, since eachSTA simultaneously transmits the PPDU, the AP, a receiver, may recognizethat the data field of the PPDU transmitted from each STA is multiplexedin the frequency domain and transmitted.

Also, in a case where both DL/UL OFDMA transmission and DL/UL MU-MIMOtransmission are supported, one resource unit may include a plurality ofstreams in a spatial domain. Also, one or more streams may be allocatedas a DL/UL spatial resource to one STA, and thus, different streams maybe allocated to a plurality of STAs.

For example, a resource unit comprised of 106 tones in FIG. 30(c)includes a plurality of streams in the spatial domain to support bothDL/UL OFDMA and DL/UL MU-MIMO.

So far, the IEEE 802.11ax WLAN system has been described. Hereinafter, aDL/UL MU data transmission method according to an embodiment of thepresent invention will be described.

DL MU Transmission of ACK Indication Information

In a case where an AP transmits a DL MU frame (that is, in a case wherethe AP DL MU-transmits a MAC frame to STAs), each STA may transmit anACK/BA frame to the AP in response to the received DL MU frame. Here,the STAs may UL SU-transmit or UL MU-transmit the ACK/BA frame. In acase where each STA UL MU-transmits the ACK/BA frame, ACK indicationinformation for UL MU transmission of the ACK/BA frame may be required.Here, the ACK indication information may indicate information for UL MUtransmission of the ACK/BA frame, a response to data transmitted througha DL MU data field. Hereinafter, for the purposes of description, theACK frame and the BA frame will be generally referred to as an “ACKframe”.

ACK indication information according to an embodiment of the presentinvention may include various types of information such as resourceallocation information, bandwidth information, channel information, MCSinformation, maximum PPDU length information, and the like.

Resource Allocation Information

It is information regarding a UL MU resource (frequency resource and/orspatial resource) allocated to each STA to UL MU-transmit an ACK frame.IN a case where the ACK is frequency-multiplexed and UL MU-transmitted,resource allocation information may include information regarding afrequency resource allocated to each STA to transmit the ACK frame.Here, the resource allocation information may include frequency resourceinformation allocated to each STA on the basis of a tone plan (pleaserefer to FIGS. 30 to 32) corresponding to a bandwidth of a UL MU PPDUcarrying the ACK frame.

In an embodiment, resource allocation information may inform each STAabout in which of resource units in a certain frequency band an ACKframe is to be UL MU-transmitted. For example, resource allocationinformation may inform STA 1 that STA 1 shall transmit an ACK frameusing first 52-tone resource unit of 20 MHz bandwidth.

In another embodiment, in a case where a resource allocation method isformed as a table like Table 13, resource allocation information mayprovide an index value corresponding to a resource allocated to eachSTA.

TABLE 13 Index Resource allcoation method 0 First 26-tone resource unit(first of 26 × 9 structure) 1 Second 26-tone resource unit (second of 26× 9 structure) . . . . . . 9 First 52-tone resource unit (first of 52 ×4 + 26 × 1 structure) . . . . . .

However, forming a table of all resource allocation methods by thebandwidths (20 MHz/40 MHz/80 MHz) may cause a problem that overhead isexcessively increased. Thus, in order to reduce overhead, a resourceallocation method of 20 MHz bandwidth may be formed as a table andresource allocation information may provide an index value correspondingto resource allocated to each STA. In this case, a bandwidth or channelinformation may be additionally provided together with the resourceallocation information. For example, in a case where 484-tone resourceunit is allocated to one STA, resource allocation information regardingthe corresponding STA may indicate a 242-tone resource unit as an indexstart value and bandwidth information may indicate 40 MHz.

In addition, resource allocation information may include informationregarding a spatial resource allocated to each STA to transmit an ACKframe.

A format of resource allocation information for UL MU transmission ofthe ACK frame may be the same as or different from a format of resourceallocation information for DL MU transmission of a DL MU frame in anHE-SIG B field.

Bandwidth Information

It is information regarding a bandwidth (20 MHz/40 MHz/80 MHz/160 MHz)of a UL MU PPDU carrying an ACK frame to be UL MU-transmitted.

Channel Information

It is information indicating whether which 20 MHz channel has beenallocated to each STA in a case where a bandwidth of a frequencyresource allocated to each STA exceeds 20 MHz. For example, in a casewhere channel information is “00”, the channel information may indicatethat a first 20 MHz channel has been allocated, and in a case wherechannel information is “01”, the channel information may indicate that asecond 20 MHz channel has been allocated.

The bandwidth information and channel information may be simultaneouslyprovided in various forms.

In an embodiment, bandwidth information and channel information may beprovided in a bitmap form. For example, in a case where bandwidthinformation and channel information regarding STA 1 is “1100”, it meansthat frequency resources of first and second 20 MHz channels among 80MHz (20 MHz*4 bits) has been allocated to STA 1. That is, the bit number(n) included in a bit map may indicate bandwidth information (20 MHz*n)and a position of a bit having a bit value 1 may indicate channelinformation.

In another embodiment, bandwidth information and channel information maybe provided in a table form. In this case, 3 bits may be required forindicating the bandwidth information and the channel information. Forexample, when bandwidth information and channel information are“000”˜“011”, it may indicate that first 20 MHz channel˜fourth 20 MHzchannel have been allocated, respectively, and “100” may indicate that a“first 40 MHz channel” has been allocated, “101” may indicate that a“second 40 MHz channel” has been allocated, and “111” may indicated thata “80 MHz channel” has been allocated.

The aforementioned embodiments are merely examples and the bandwidthinformation and channel information may be configured as variousembodiments.

MCS Information

It is information regarding an MCS level applied to an ACK frame to beUL MU-transmitted.

The MCS information may have a bit size of 4 to 5 bits and directlyindicate an MCS level applied to an ACK frame. In this case, the MCSinformation may indicate every MCS level defined in a system.

Or, when it is assumed that a lower MCS level is applied to a UL

MU-transmitted ACK frame for robust transmission, MCS information mayindicate a difference value regarding an MCS level applied to an ACKframe and an MCS level applied to a DL MU frame. For example,

When MCS information is “00”, it may indicate that the same MCS level asthat of a DL MU frame is applied to an ACK frame,

When MCS information is “01”, it may indicate that an MCS level one-steplower than that applied to a DL MU frame is applied to an ACK frame,

When MCS information is “10”, it may indicate that an MCS level two-steplower than that applied to a DL MU frame is applied to an ACK frame, and

When MCS information is “11”, it may indicate that an MCS levelthree-step lower than that applied to a DL MU frame is applied to an ACKframe.

Here, the higher/lower MCS level may refer to an MCS level indicating amodulation scheme in which a data bit number per symbol isgreater/smaller, or may refer to an MCS level indicating a higher/lowercode rate in a case where a modulation scheme is the same. A lower MCSlevel is more advantageous for robust transmission.

Or, MCS information may selectively indicate only a low MCS level forrobust transmission, irrespective of an MCS level applied to a DL MUframe. For example,

When MCS information is “00’, it may indicate an MCS level 0 (e.g., BPSKmodulation and 1/2 code rate coding),

When MCS information is “01’, it may indicate an MCS level 1 (e.g., QPSKmodulation and 1/2 code rate coding),

When MCS information is “10’, it may indicate an MCS level 2 (e.g., QPSKmodulation and 3/4 code rate coding), and

When MCS information is “11’, it may indicate an MCS level 3 (e.g.,16QAM modulation and 1/2 code rate coding).

Maximum length information of PPDU (or maximum length information of ACKframe)

It is maximum length information of a UL MU PPDU carrying an ACK frame.Or, it is length information of an ACK frame having a longest lengthamong ACK frames carried by a UL MU PPDU.

ACK frames transmitted by each STA may have different lengths accordingto UL MU frequency resources used for UL MU transmission and applied MCSlevels, but in order to prevent interference, all the ACK frames may bepadded to have the same length and transmitted. Thus, in a case whereeach STA pads a UL MU PPDU (and/or ACK frame) to be transmitted by eachSTA such that a length thereof is the same as a maximum length of a ULMU PPDU (and/or maximum length information of the ACK frame), each STAmay require maximum length information of the UL MU PPDU (and/or maximumlength information of the ACK frame). The maximum length of the UL MUPPDU may be expressed by a micro second (us) unit or a symbol numberunit.

In case where the maximum length of the UL MU PPDU is expressed by asymbol number unit, signaling overhead regarding the symbol number maybe reduced by calculating the symbol number excluding 40 us of an alwaysincluded physical preamble.

For example, a length of a UL MU PPDU, to which BPSK modulation and anMCS level of a 1/2 code rate are applied, and which is transmitted usinga 26-tone resource unit, is about 400 us, and here, when the symbolnumber is calculated excluding a physical preamble, it is about 26symbols. When the maximum length of the UL MU PPDU is considered, a bitsize of maximum length information of the UL MU PPDU may be 5 bits(00000: 1 symbol˜11111: 32 symbol)

Others

In addition to the aforementioned information, various types of triggerinformation for UL MU transmission of an ACK frame such as buffer statusreport information, channel status report information, triggerinformation for random access of STAs, cyclic prefix (CP) lengthinformation, whether an STBC is used, a coding method, and the like, maybe included.

These information items may be signaled in a preset manner according toan embodiment or may be signaled in the same manner as that of asignaling method of a DL MU frame. Also, the aforementioned informationmay be selectively included in ACK indication information and, besidesthe aforementioned information, additional information may be includedin the ACK indication information.

Hereinafter, a method for transmitting the aforementioned ACK indicationinformation will be described with reference to FIGS. 33 and 34.

A method for transmitting ACK indication information may be classifiedinto two types as follows.

1. ACK indication information is included in physical preamble andtransmitted

2. ACK indication information is included in data field and transmitted

An embodiment in which the ACK indication information is included in aphysical preamble and transmitted will be described with reference toFIG. 33 and an embodiment in which the ACK indication information isincluded in a data field and transmitted will be described withreference to FIGS. 34 to 36.

FIG. 33 is a diagram illustrating an embodiment of a 20 MHz DL MU PPDUin which ACK indication information is included in a physical preamble.

Referring to FIG. 33, the 20 MHz DL MU PPDU may include a physicalpreamble and a data field following the physical preamble. In detail,the 20 MHz DL MU PPDU may be configured in order of L-STF field→L-LTFfield→L-SIG field→RL-SIG (Repeated L-SIG) field→HE-SIG A field→HE-SIG Bfield→HE-STF field→HE-LTF field→HE-SIG C field. The order of the fieldsmay be changed according to an embodiment, a specific field may beadded, and some fields may not be included.

The ACK indication information may be included in a HE-SIG B field or aHE-SIG C field of a physical preamble.

1. When Included in HE-SIG B Field

The ACK indication information may be included in the HE-SIG B field ofa physical preamble and DL MU-transmitted. Here, the HE-SIG B field mayinclude “common information (or a common field)” commonly required byreception STAs of a DL MU PPDU (DL MU-transmitted PPDU) and “userspecific information (or a user specific field) individually requiredfor reception STAs.

In an embodiment, the ACK indication information may be included in thecommon information or the user specific information of the HE-SIG Bfield. For example, in a case where the ACK indication informationincludes indication information regarding the entirety of receptionSTAs, the corresponding ACK indication information may be included inthe common information of the HE-SIG B field. Or, in a case where theACK indication information includes indication information for eachreception STA, the corresponding ACK indication information may beincluded in the user specific information of the HE-SIG B field.

In another embodiment, the ACK indication information may be included incommon information and user specific information of the HE-SIG B field.In detail, an ACK sub-indication information regarding the entirety ofreception STAs included in the ACK indication information may beincluded in the common information and ACK sub-indication information ofeach reception STA included in the ACK indication information may beincluded in the user specific information.

For example, UL MU resource allocation information (i.e., sub-indicationinformation) of the entirety of reception STAs for ACK frametransmission may be included in the common information. Or, UL MUresource allocation information (or sub-indication information) for eachSTA for ACK frame transmission may be included in the user-specificinformation. Or, information regarding an MCS level applied to an ACKframe by reception STAs may be included in the user specificinformation. Or, when the MCS levels applied to the reception STAs arethe same, information regarding the corresponding MCS level may beincluded in the common information. Or, when it is assumed that ACKframes respectively transmitted from the reception STAs are padded tohave the same length, maximum length information (i.e., sub-indicationinformation) of the ACK frame may be included in the common information.

In addition, the sub-indication information included in the ACKindication information may be included in the common information or theuser specific information according to characteristics, without beinglimited to the aforementioned embodiment.

2. When Included in HE-SIG C Field

The ACK indication information may be included in an HE-SIG C field of aphysical preamble and DL MU-transmitted.

As mentioned above, in the case of 20 MHz DL MU PPDU, 64 FFTs are usedup to HE-SIG B field, and 256 (4*64) FFTs may be used from the HE-STF.In this case, each STA may obtain information regarding DL MU resourceallocated to each STA using DL MU resource allocation informationincluded in the HE-SIG B field. Since the HE-SIG C field is positionedbehind the HE-SIF, it may be transmitted using resource separatelyallocated to each STA, and thus, the HE-SIG C field may be used fortransmitting information specific to each STA. Thus, the AP may DLMU-transmit ACK indication information for ACK frame transmission ofeach STA in the HE-SIG C field.

In this manner, the ACK indication information included in the physicalpreamble may be DL MU-transmitted to each STA through the DL MU PPDU,and each STA may UL MU-transmit an ACK frame using the UL MU resourceallocated thereto according to the received ACK indication information.

FIG. 34 is a diagram illustrating an embodiment of a 20 MHz DL MU PPDUin which ACK indication information is included in a data field. Thefields illustrated in FIG. 34 are the same as those described above withreference to FIGS. 6, 7 and 17, and thus, repeated descriptions thereofwill be omitted.

ACK indication information may be included in a data field in variousembodiments. For example, in a case where the data field includes anA-MPDU, the ACK indication information may be included in a MAC headerof at least one MPDU included in the A-MPDU. Or, the ACK indicationinformation may be included in a MAC frame body of at least one MPDUincluded in the A-MPDU. Hereinafter, an embodiment in which the ACKindication information is included in an MAC header will be described.

In an embodiment, the ACK indication information may be included in aframe control field included in the MAC header. In the related art, anoption in which bit values of a To DS field and From DS field includedin the frame control field are 1 is used to indicate a mesh BSS.However, in the present invention, such an option may be used as anindicator indicating whether ACK indication information is included inthe MAC header. Thus, in a case where the To DS field and From DS fieldvalues are set to 1 (that is, in a case of indicating that the ACKindication information is included), an address 4 field (6 octets) maybe used as a field for transmitting ACK indication information.

In another embodiment, the ACK indication information may be included inthe control field included in the MAC header. In detail, the ACKindication information may be included in an HT control field (4 octets)included in a MAC header of an HT format MPDU. Or, the ACK indicationinformation may be included in an HE control field included in a MACheader of an HE format MPDU newly defined in a 802.11ax system.

Here, the HE control field (4 octets) may be a field in which the HTcontrol field is newly configured to fit the 802.11x system. Or, the HEcontrol field (10 octets) may be a field newly configured by adding theaforementioned address 4 field (6 octets) and the HT control field (4octets). Or, the HE control field may be a field newly configured tohave a size of 4 to 6 octets in the place of the HT control fieldconsisting of 4 octets in an HT format.

As described above, the ACK indication information may be included invarious fields included in the MAC header in various embodiments. Inthis case, the MAC header may additionally include an indicatorindicating that it includes the ACK indication information. Hereinafter,various embodiments of the indicator will be described.

Indicator Indicating that ACK Indication Information is Included

The A-MPDU subframe may include an indicator indicating that a MACheader included therein includes ACK indication information. Theindicator may be included in the subframe of the A-MPDU in variousembodiments.

1) First Embodiment

The indicator may be included in an MPDU delimiter corresponding to aMAC header including ACK indication information. In detail, an MPDUdelimiter field included in one A-MPDU subframe may include an indicatorindicating that the MAC header included in the A-MPDU subframe includesthe ACK indication information. For example, a specific bit (1 bit)among reserved bits (4 bits) included in the MPDU delimiter field mayserve as an indicator. In a case where the specific bit value serving asan indicator is set to a preset value (e.g., 1), it may indicate that aMAC header including the indicator includes the ACK indicationinformation.

Also, the indicator included in the MPDU delimiter field mayadditionally indicate that an MPDU corresponding to the MPDU delimiterfield includes an HE control field newly defined in the 802.11ax SYSTEM.In detail, a MPDU delimiter field included in one A-MPDU subframe mayadditionally indicate that the MPDU included in the A-MPDU subframeincludes the HE control field. Here, the ACK indication information maybe included in the HE control field. In this case, the MPDU includingthe corresponding HE control field may not correspond to an HT controlwrapper frame.

2) Second Embodiment

In a case where a type or a sub-type of the MPDU including the ACKindication information is newly defined in the 802.11ax system, theindicator may be included in the frame control field of the MAC headerof the MPDU, as a newly defined type or sub-type. That is, the type orsub-type of the MPDU may indicate, as an indicator, that thecorresponding MPDU is an MPDU including ACK indication informationwithin a MAC header. In this case, the type or sub-type as the indicatormay be newly defined in the 802.11ax system.

3) Third Embodiment

In a case where the MPDU including the ACK indication informationcorresponds to an HT format frame (e.g., an HT control wrapper frame)including the HT control field, a specific bit within the HT controlfield may serve as an indicator. In detail, a specific bit amongreserved bits included in the HT control field may serve as anindicator, and in a case where the corresponding specific bit is set bya preset value, it may indicate that the corresponding HT control fieldincludes the ACK indication information.

4) Fourth Embodiment

FIG. 35 is a drawing illustrating a control field of an HT format.Referring to FIG. 35, a first bit of the control field may serve as anindicator indicating that a corresponding control field is a VHT controlfield of a VHT format. For example, in a case where a value of a firstbit of the control field is set to a preset value (e.g., 1), it mayindicate that the corresponding control field is a VHT control field.

Similarly, a second bit (reserved bit) 3510 of the HT control field mayinclude ACK indication information, and may serve as an indicatorindicating that the corresponding control field is an HE control fieldof an HE format newly defined in the 802.11ax system. For example, in acase where a first bit of the control field indicates a VHT format and asecond bit 3510 indicates inclusion of the ACK indication information(or an HE format) (or in a case where the second bit 3510 is set to apreset value (e.g., 1), it may indicate that the corresponding field isan HE control field including the ACK indication information.

The HE control field may be newly configured using an HT control field,and details of a configuration of the HE control field will be describedwith reference to FIG. 36.

5) Fifth Embodiment

The indicator may be included in a specific field among fields includedin the MAC header. In detail, the indicator may be included in aspecific field that can be reinterpreted among the fields included inthe MAC header.

For example, as described above with reference to FIG. 34, In therelated art, an option in which bit values of a To DS field and From DSfield included in the frame control field are 1 is used to indicate amesh BSS. However, in the present invention, such an option may be usedas an indicator indicating whether ACK indication information isincluded in the MAC header. Thus, in a case where the To DS field andFrom DS field values are set to 1 (that is, in a case of indicating thatthe ACK indication information is included), an address 4 field (6octets) may be used as a field for transmitting ACK indicationinformation. In this case, the To DS field and From DS field may serveas an indicator, and, through the To DS field and From DS field, it maybe indicated that the MAC header including the fields include the ACKindication information.

Or, in another example, a specific bit (e.g., 12^(th) bit (B12) from themost significant bit (MSB) of an AID field) included in a specificaddress field included in the MAC header may serve as an indicator.

So far, various embodiments of the indicator have been described. Theindicator may be included in the MAC header in various embodiments toindicate that a corresponding MAC header includes the ACK indicationinformation, without being limited to the aforementioned embodiment.

Hereinafter, a configuration of the HE control field described above inrelation to the fourth embodiment will be described in detail.

FIG. 36 is a diagram illustrating an HE control field according to anembodiment of the present invention.

Descriptions of some fields included in the HT control field (describedabove with reference to FIG. 8) may also be applied in the same mannerto some fields included in the HE control field in FIG. 36. Thus, onlydifferences from the HT control field will be described. The fieldsillustrated in FIG. 36 may be independently present, may be selectivelyincluded in the HE control field, and order of the fields and bit sizesmay be modified according to embodiments.

Referring to FIG. 36, unlike the HT control format, the HE control fieldmay include an indicator and ACK indication information. Also, asdescribed above, a first bit of the HE control field may indicate thatthe corresponding control bit is in a VHT format and a second bit mayindicate that the corresponding control bit is in an HE format.

FIG. 36(a) is a drawing illustrating a DL MU-transmitted HE controlfield.

Referring to FIG. 36(a), the HE control field may include at least oneof ACK channel information for UL MU transmission of the ACK frame,butter status report request information (1 bit) 3630, and channelstatus report (1 bit) (not shown). Here, the ACK channel informationrefers to resource allocation information as information regarding UL MUresource allocated to each STA to UL MU-transmit an ACK frame, and thedescriptions and embodiment of the resource allocation informationdescribed above may be applied in the same manner. Here, the ACK channelinformation may include ACK channel start index information (4 bits)3610 and ACK channel duration information (4 bits) 3620 as anotherembodiment of the aforementioned resource allocation information.

When it is assumed that “one ACK channel” is a minimum allocation unitof UL MU resource for the ACK frame, start index information 3610 of theACK channel and duration information 3620 of the ACK channel indicatinga size (or the number) of the ACK channel may be required. In this case,bit sizes of the start index information 3610 and the durationinformation 3620 of the ACK channel may be determined on the basis of amaximum number of STAs (or users) to which the DL MU resource may beallocated. For example, in a case where the maximum number of STAs towhich the DL MU resource (including a frequency resource and a spaceresource) can be allocated is 16 (2̂4 bits), the start index information3610 and the duration information 3620 of the ACK channel may have a bitsize of 4 bits.

In a case where the sizes (or numbers) of the ACK channels respectivelyallocated to the STAs are set to be the same (that is, in a case wherethe MCS level applied to the ACK frame is fixed), the durationinformation 3620 of the ACK channel may not be required. Or, in a casewhere an MCS level determined on the basis of an MCS level applied to aDL frame is applied to the ACK frame, ACK channel information mayindicate a difference value between the MCS level applied to a DL frameand the MCS level applied to the ACK frame. For example, the ACK channelinformation may indicate “−2, −1, 0, 1” as a difference value(specifically, the MCS level applied to the ACK frame—the MCS levelapplied to the DL frame) between the MCS level applied to the ACK frameand the MCS level applied to the DL frame, and may be signaled by a bitsize of 2 bits.

In the aforementioned embodiment, it is assumed that a size (or thenumber) of the ACK channel is fixed according to an MCS level applied tothe ACK frame.

In addition, the HE control field may further include variousinformation items (e.g., bandwidth information, MCS information, maximumPPDU length information, and the like) included in the aforementionedACK indication information.

FIG. 36(b) is a drawing illustrating a UL MU-transmitted HE controlfield.

Referring to FIG. 36(b), the HE control field may include at least oneof buffer status information (8 bits) (3640-1, 3640-2), whether there iscontents for reporting a butter status, and channel information. Thebuffer status information may include two fields, and a bit size of eachfield may 4 bits. The buffer status information may indicate a queuesize, an access category (AC), a backoff count, and the like.

A type indicator indicating a type of information that may be includedin the HE control field may be included in a field ahead of the HEcontrol field (or preceding field).

Error Recovery

In a case where a data field of a DL MU PPDU is configured as anAN-MPDU, the AP may include ACK indication information in each MACheader of every MPDU forming the A-MPDU or may include ACK indicationinformation in a MAC header of a partial MPDU (e.g., a first MPDU amongMPDUs forming the A-MPDU).

In a case where the AP includes the ACK indication information in eachMAC header of every MPDU forming the A-MPDU, repeated information isincluded in every MPDU, disadvantageously increasing overhead.Conversely, in a case where the AP includes the ACK indicationinformation in the MAC header of a partial MPDU, overhead may be reducedbut if a STA fails to decode the corresponding partial MPDU, it cannotUL MU-transmit an ACK frame. In order to prevent such a problem, in acase where the ACK indication information is included in the MAC headerof the partial MPDU, a predetermined error recovery procedure may berequired. Hereinafter, the error recovery procedure according to anembodiment of the present invention will be described in detail.

FIG. 37 is a drawing schematically illustrating an error recoveryprocedure according to an embodiment of the present invention.Hereinafter, it is assumed that an AP transmits a DL MU frame (or DLdata) to STAs 1 to 4 and the STAs 1 to 4, upon receiving the DL MU frame(or DL data) UL MU-transmits an ACK frame in response to the received DLMU frame (or DL data).

Referring to FIG. 37, the AP may transmit a DL MU frame (or DL data) tothe STAs 1 to 4 using DL MU resource. In this case, ACK indicationinformation for UL MU transmission of an ACK frame may be included in apartial MAC header of the DL MU frame (or DL data) transmitted to eachSTA.

When the STAs 1 and normally receive the DL MU frame (or DL data), theSTAs 1 and 2 each may UL MU-transmit an ACK frame in response to the DLMU frame (or DL data) after the lapse of a predetermined time. In thiscase, the STAs 1 and 2 may UL MU-transmit the ACK frame using a UL MUresource indicated by the ACK indication information included in thepartial MAC header of the received DL MU frame (or DL data).

The STAs 3 and 4, which have not normally received the DL MU frame (orDL data) or the ACK indication information, cannot UL MU-transmit an ACKframe in response to the DL MU frame (or DL data).

In this case, after the AP UL MU-receives the ACK frames from the STAs 1and 2, it may DL MU-transmit an MU BAR frame to the STAs 3 and 4 whichhave failed to UL MU-transmit an ACK frame after the lapse of apredetermined time (e.g., SIFS). Or, after performing a backoffprocedure to re-transmit the DL MU frame (or DL data) to the STAs 3 and4, the AP may DL MU-transmit the MU BAR frame through channelcontention. The MU BAR frame may include STA IDs of the STAs which havefailed in UL MU transmission of an ACK frame and new indicationinformation for UL MU transmission of an ACK frame.

In this case, the STAs 3 and 4 obtain an opportunity to UL MU-transmitan ACK frame. On the basis of the received MU BAR, the STAs 3 and 4 mayUL MU-transmit an ACK frame.

If the STAs 3 and 4 do not UL MU-transmit the ACK frame even after theAP has transmitted the MU BAR, the AP may determine that the STAs 3 and4 have not properly decoded the DL MU frame (or DL data), and perform afollow-up procedure such as re-transmission of the DL MU frame (or DLdata), or the like.

In another embodiment, STAs, which have received the DL MU frame (or DLdata) but have not UL MU-transmitted an ACK frame because they failed toreceive ACK indication information, may UL SU-transmit an ACK framethrough channel contention. Here, the ACK frame may be a stand-alongframe included solely in a UL frame or may be a frame piggybacked to adata frame of the UL frame.

In this case, since the AP cannot keep waiting for receiving an ACKframe from the STAs, a predetermined waiting time may be set and the APmay wait for UL SU-receiving an ACK frame only for the waiting time. Ina case where ACK frames are not received from the corresponding STAs forthe waiting time, the AP may determine that the STAs which have nottransmitted the ACK frames are STAs which have not properly decoded a DLMU frame (or DL data), and perform a follow-up procedure such asre-transmission of a DL MU frame (or DL data).

Or, the AP may receive a BAR request and/or ACK frame from the STAswhich has failed to UL MU-transmit an ACK frame, during a backoffprocedure for re-transmitting the DL MU frame (or DL data) or in thecourse of transmitting an MU BAR frame. In this case, the AP may performthe follow-up procedure such as re-transmission of the DL MU frame (orDL data) and/or transmission of the BAR frame on the basis of a receivedBAR request and/or ACK frame.

In another embodiment, STAs, which have received a DL MU frame (or DLdata) but failed to UL MU-transmit an ACK frame because they had failedto receive ACK indication information, may request a BAR frame from theAP during a random access interval (e.g., a next random access intervalor an random access interval designated by the AP) or directly transmitan ACK frame to the AP.

Here, since the AP cannot keep waiting for receiving an ACK frame or aBAR frame request, a predetermined waiting time may be set and the APmay wait for receiving an ACK frame or a BAR frame request only for thewaiting time. In a case where an ACK frame or a BAR frame request is notreceived from the corresponding STAs for the waiting time, the AP maydetermine that the STAs which have not transmitted an ACK frame are STAswhich had not properly decoded a DL MU frame (or DL data), AND perform afollow-up procedure such as re-transmission of the DL MU frame (or DLdata).

Or, the AP may receive a BAR request and/or ACK frame from the STAswhich have failed to UL MU-transmit an ACK frame during a backoffprocedure for re-transmitting the DL MU frame (or DL data) or in thecourse of transmitting a MU BAR frame. In this case, the AP may performa follow-up procedure such as retransmission of the DL MU frame (or DLdata) and/or BAR frame transmission on the basis of the received BARrequest and/or ACK frame.

FIG. 38 is a flow chart illustrating a DL MU transmission method of anAP device according to an embodiment of the present invention. Theaforementioned embodiments may be applied in the same manner in relationto the flow chart. Thus, hereinafter, repeated descriptions will beomitted.

Referring to FIG. 38, an AP may generate a DL MU PPDU (S3810). Here, theDL MU PPDU may include a physical preamble and a data field. The datafield may include at least one MPDU, and here, the at least one MPDU mayinclude a MAC header and a MAC frame body. Also, the MAC header includesACK indication information for UL MU transmission of an ACK frame as aresponse to data transmitted through the data field.

Thereafter, the AP may transmit a DL MU PPDU (S3820). In detail, the APmay transmit a DL MU PPDU to at least one STA which has been allocated aDL resource.

FIG. 39 is a block diagram of each STA device according to an embodimentof the present invention.

In FIG. 39, an STA device 3910 may include a memory 3912, a processor3911 and an RF unit 3913. And, as described above, the STA device may bean AP or a non-AP STA as an HE STA device.

The RF unit 3913 may transmit/receive a radio signal with beingconnected to the processor 3911. The RF unit 3913 may transmit a signalby up-converting the data received from the processor 3911 to thetransmission/reception band.

The processor 3911 may implement the physical layer and/or the MAC layeraccording to the IEEE 802.11 system with being connected to the RF unit4013. The processor 3911 may be constructed to perform the operationaccording to the various embodiments of the present invention accordingto the drawings and description. In addition, the module forimplementing the operation of the STA 3910 according to the variousembodiments of the present invention described above may be stored inthe memory 3912 and executed by the processor 3911.

The memory 3912 is connected to the processor 3911, and stores varioustypes of information for executing the processor 3911. The memory 3912may be included interior of the processor 3911 or installed exterior ofthe processor 3911, and may be connected with the processor 3911 by awell known means.

In addition, the STA device 3910 may include a single antenna or amultiple antenna.

The detailed construction of the STA device 3910 of FIG. 39 may beimplemented such that the description of the various embodiments of thepresent invention is independently applied or two or more embodimentsare simultaneously applied.

The embodiments described above are constructed by combining elementsand features of the present invention in a predetermined form. Theelements or features may be considered optional unless explicitlymentioned otherwise. Each of the elements or features can be implementedwithout being combined with other elements. In addition, some elementsand/or features may be combined to configure an embodiment of thepresent invention. The sequential order of the operations discussed inthe embodiments of the present invention may be changed. Some elementsor features of one embodiment may also be included in anotherembodiment, or may be replaced by corresponding elements or features ofanother embodiment. Also, it will be obvious to those skilled in the artthat claims that are not explicitly cited in the appended claims may bepresented in combination as an exemplary embodiment of the presentinvention or included as a new claim by subsequent amendment after theapplication is filed.

The embodiments of the present invention may be implemented throughvarious means, for example, hardware, firmware, software, or acombination thereof. When implemented as hardware, one embodiment of thepresent invention may be carried out as one or more application specificintegrated circuits (ASICs), one or more digital signal processors(DSPs), one or more digital signal processing devices (DSPDs), one ormore programmable logic devices (PLDs), one or more field programmablegate arrays (FPGAs), a processor, a controller, a microcontroller, amicroprocessor, etc.

When implemented as firmware or software, one embodiment of the presentinvention may be carried out as a module, a procedure, or a functionthat performs the functions or operations described above. Software codemay be stored in the memory and executed by the processor. The memory islocated inside or outside the processor and may transmit and receivedata to and from the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above exemplary embodiments are therefore to beconstrued in all aspects as illustrative and not restrictive. The scopeof the invention should be determined by the appended claims and theirlegal equivalents, not by the above description, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein

INDUSTRIAL APPLICABILITY

While a frame transmission scheme in a wireless communication systemaccording to the present invention has been described with respect toits application to an IEEE 802.11 system, it also may be applied toother various wireless communication systems than the IEEE 802.11system.

1-18. (canceled)
 19. A downlink (DL) multi-user (MU) transmission methodin a wireless local area network (WLAN) system, the DL MU transmissionmethod comprising: generating a DL MU physical protocol data unit (PPDU)including a physical preamble and a data field, wherein the data fieldincludes at least one MAC protocol data unit (MPDU), the at least oneMPDU includes a MAC header and a MAC frame body, and wherein the MACheader includes ACK indication information for uplink (UL) MUtransmission of an ACK frame as a response to data transmitted throughthe data field; and transmitting the DL MU PPDU.
 20. The DL MUtransmission method of claim 19, wherein the MAC header includes anindicator indicating whether the ACK indication information is includedor not.
 21. The DL MU transmission method of claim 20, wherein theindicator is included in an HE control field included in the MAC header.22. The DL MU transmission method of claim 21, wherein the ACKindication information is included in the HE control field of the MACheader.
 23. The DL MU transmission method of claim 22, wherein the ACKindication information includes frequency resource allocationinformation, modulation and coding scheme (MCS) level information andlength information of a UL MU PPDU carrying the ACK frame for UL MUtransmission of the ACK frame.
 24. The DL MU transmission method ofclaim 23, wherein the frequency resource allocation information includesan index value indicating a resource unit allocated for UL MUtransmission of the ACK frame.
 25. The DL MU transmission method ofclaim 23, wherein the MCS level information indicates an MCS levelapplied to the ACK frame.
 26. The DL MU transmission method of claim 25,wherein the MCS level applied to the ACK frame is selected amongpredetermined number of lowest MCS levels.
 27. The DL MU transmissionmethod of claim 26, wherein the MCS level information indicates one ofMCS levels 0 to 3, wherein the MCS level 0 indicates BPSK modulation and1/2 code rate coding, the MCS level 1 indicates QPSK modulation and 1/2code rate coding, the MCS level 2 indicates QPSK modulation and 3/4 coderate coding, and the MCS level 3 indicates 16QAM modulation and 1/2 coderate coding.
 28. The DL MU transmission method of claim 23, wherein thelength information indicates a length of the UL MU PPDU in an OrthogonalFrequency Division Multiple (OFDM) symbol number unit.
 29. A Station(STA) device in a wireless local area network (WLAN) system, the STAdevice comprising: a radio frequency (RF) unit configured to transmitand receive a wireless signal; and a processor configured to control theRF unit; wherein the processor is further configured to: generate adownlink (DL) multi-user (MU) physical protocol data unit (PPDU)including a physical preamble and a data field and transmits the DL MUPPDU, wherein the data field includes at least one MAC protocol dataunit (MPDU), the at least one MPDU includes a MAC header and a MAC framebody, and wherein the MAC header includes ACK indication information foruplink (UL) MU transmission of an ACK frame as a response to datatransmitted through the data field, and transmits the DL MU PPDU. 30.The STA device of claim 29, wherein the MAC header includes an indicatorindicating whether the ACK indication information is included or not.31. The STA device of claim 30, wherein the indicator is included in anHE control field included in the MAC header.
 32. The STA device of claim31, wherein the ACK indication information is included in the HE controlfield of the MAC header.
 33. The STA device of claim 32, wherein the ACKindication information includes frequency resource allocationinformation, modulation and coding scheme (MCS) level information andlength information of a UL MU PPDU carrying the ACK frame for UL MUtransmission of the ACK frame.
 34. The STA device of claim 33, whereinthe frequency resource allocation information includes an index valueindicating a resource unit allocated for UL MU transmission of the ACKframe.
 35. The STA device of claim 33, wherein the MCS level informationindicates an MCS level applied to the ACK frame.
 36. The STA device ofclaim 35, wherein the MCS level applied to the ACK frame is selectedamong predetermined number of lowest MCS levels.
 37. The STA device ofclaim 36, wherein the MCS level information indicates one of MCS levels0 to 3, wherein the MCS level 0 indicates BPSK modulation and 1/2 coderate coding, the MCS level 1 indicates QPSK modulation and 1/2 code ratecoding, the MCS level 2 indicates QPSK modulation and 3/4 code ratecoding, and the MCS level 3 indicates 16QAM modulation and 1/2 code ratecoding.
 38. The STA device of claim 33, wherein the length informationindicates a length of the UL MU PPDU in an Orthogonal Frequency DivisionMultiple (OFDM) symbol number unit.